Sacred status of hadith/sunnah in Islam
The primary sources of revelation in Islam are the Quran and Hadith/Sunnah. Quran explicitly orders the Muslims to obey and follow the Prophet's teaching and example when learning the book: "Allah did confer a great favour on the Believers when He sent among them a Messenger from among themselves, who rehearses to them the Signs of Allah, purifies them, instructs them in Scripture, and teaches them Wisdom, whereas previously they had been in plain error."  Here, and many other places in the Quran, we are informed about the importance of Hadith & Sunnah because the entire practice of the Quran and how to live by it is found in the life of the Prophet ﷺ. Since Allah ﷻ promises in ‘Al Quran 15:09’ that He has protected the message of guidance from all corruption till the day of judgment therefore His commandment to follow Prophetic teachings also hold exact same surety of protection.
The role of Isnad in hadith preservation
The hadith literature is a sacred tradition alongside Quran and the key to this sacredness lies in something called Isnad which basically means "chain of transmission". An Isnad contains a list of narrators who memorized and transmitted a hadith through a chain, that directly reaches all the way back to the sacred person of the Prophet ﷺ. For example lets say Imam Zaid memorized and wrote down a hadith he heard from his teacher Imam, who learned and memorized from his teacher a Companion, who directly met and heard from the Prophet ﷺ. Now when Imam Zaid will go and teach it to his students they will memorize the tradition, write it down and in turn will teach to their students. Through this process not only the hadith got memorized by who were taught but also transmitted from generation to generation, oral and in writing, linked with a chain that reaches back all the way to the Prophet’s sacred personality, who himself was recipient of revelation from Allah ﷻ through Angel Jibraeel (alaihi salaam). But there is one major sacred & spiritual factor guiding the isnad process. Nazim Bakhsh, quotes in his writing Shaykh Hamza Yusuf, a leading Islamic scholar, who gives an overview of how scholars have made use of Isnad process from generation to generation through something called Ijaza: "Shaykh Hamza Yusuf, has often said that isnad is the secret of this Ummah and a gift from Allah. Without isnad the entire tradition could very well collapse. The system of ijaza (teaching licenses) is intricately linked to isnad in that one takes his knowledge from noble men and women who took their knowledge from those who took their knowledge from those….all the way back to that model community (companions) and to the blessed Messenger himself, whose knowledge, without a shadow of doubt, came from the Lord of the Divine Throne through his messenger, the Angel Gibril (Gabriel), upon him be peace.”  Ijazas are teaching licenses given by a Sheikh, well versed and knowledgeable in Islam to those of his students whom he approves to be fit of transmitting hadith and other knowledge directly from himself. This has maintained the transmission process called isnad not only on the academic level but also on a spiritual level.
As discussed earlier, Isnad is part of a wider process called ilm al hadith (sciences of hadith), that maintains the unanimous position of the Islamic scholars who not only recorded & memorized the hadith but also scrutinized them making it clear which of them fall under category of sahih (authentic), mursal (interrupted), dhaeef (weak), munkar (rejected), mawdoo (fabricated), in terms of their chain of narrators. They were familiar with who the forgers were and who were the unknown narrators, who should not be considered fully trustworthy in the chain. All of that is due to the position and importance of the Prophet Muhammad ﷺ, as the seal of all the Prophets and leader of humankind. Hence it is these sacred factors that have guided the systematic process of hadith collection and preservation due to which forgeries and interpolations have not been able to be attributed to him and to be accepted by the majority consensus of Islamic scholars.
 Al Quran 3:164 - Asad Translation
 Baksh, Nazim. 2015. In The Spirit of Tradition. January 5th. Accessed January 7th, 2020. http://masud.co.uk/in-the-spirit-of-tradition/.
There has been discovery of several hadith texts belonging to the 1st Century Hijrah (Islamic year) whose contents were compared by modern Muslim & Non-Muslim scholars and researchers, with the canonical collections of the 3rd century Hijrah (9th century CE) of Imam Bukhari, Imam Muslim, Imam Ahmad etc. Their findings reveal that the hadith were almost identical with no issues. Among those that were subject to such research in the recent years in particular were Sahihfa (scriptures) of Hamam ibn Munabbih and Musannaf of Abdul Razzaq of the 1st Century. Below are some of their findings which support the fact that not only were hadith being collected in the first century but also their authenticity has been proven to be correct.
Sahifah (Scripture of) Hamam Ibn Munabbih
Sahifah (Scripture of) Hammam ibn Munabbih (d. 101 AH) is perhaps one of the earliest known hadith collections, by 8th century scholar Hammam ibn Munabbih. It has been translated in the 20th century by Muhammad Hamidullah - a world-renowned Muhaddith, Faqih and scholar of Islam and International Law from Pakistan, winner of national award Hilal-e-Imtiaz in Pakistan. Hammam ibn Munabbih was the taba'een (student) of the Companion, Abu Huraira (r.a), the famous hadith transmitter. Dr Hamidullah published this Sahifa of Hammam around 1979 and it proves that hadith were collected in the Mid 1st century Hijrah. His research was also compared by orientalists such as, by R. M. Speight who mentions that: “… the texts in Hammam and those recorded in Ibn Hanbal, Bukhari and Muslim with the same isnad show almost complete identity, except for a few omissions and interpolations which do not affect the sense of the reports. On the other hand, the same ahadith as told by other transmitters in the three collections studied show a rich variety of wording, again without changing the meaning of the reports.”  Speight shared his analysis after comparing it (i.e., the Sahifa) with about the 1500 variant readings of the same ahadith found in the collections of Ibn Hanbal (Musnad), al-Bukhari (Sahih) and Muslim (Sahih); dating from 3rd Century Hijrah.
Musannaf Abdul Razzaq
Another book from the 1st century belonging to the famous early Muslim Muhaddith (hadith expert and teacher) named Abdul Razzaq Sanani (124 AH - 211 AH) was studied by another Western Non-Muslim writer name Harald Motzki. His analysis appeared in the Journal of Near Eastern Studies and he says that: “The works of ‘Abd al-Razzak are extremely important for the study of early Islamic jurisprudence, hadith and exegesis of the Kur’an because they contain older sources or materials which have otherwise been lost. ‘Abd al-Razzak had direct access to authors of the first extensive compilations of traditions arranged according to subject . . . like those by Ma’mar b. Rashid, Ibn Djuraydj, Sufyan al-Thawri and Sufyan b. ‘Uyayna. His own Musannaf is to a large extent compiled from materials received from these four scholars, and it is very probable that these materials came for the most part from their books. In general, ‘Abd al-Razzak’s transmission from these teachers of his seems to be reliable . . . While studying the Musannaf of `Abd al-Razzaq, I came to the conclusion that the theory championed by Goldziher, Schacht, and in their footsteps, many others - myself included - which in general, reject hadith literature as a historically reliable sources for the first century AH, deprives the historical study of early Islam of an important and a useful type of source.” 
Even though as Muslim Believers we do not need the above findings to prove us the authenticity of the hadith literature, yet this confirms for us even more that the hadith of the Prophet ﷺ were collected through a science called ilm Al hadith which no doubt was an effective methodology and has produced for us results that helped Muslims guided for the past 14 centuries.
 R. M. Speight, “A Look At Variant Readings In The Hadith”, Der Islam, 2000, Band 77, Heft 1, p. 170 https://www.islamic-awareness.org/hadith/hadith
 H. Motzki, “al-Sanani, Abd al-Razzak b. Hammam b. Nafi, Abu Bakr al-Yamani al Himyari (IX 7a)”, in Encyclopaedia of Islam CD-ROM Edition v. 1.0
Originally written on July 12, 2020 on Quora - Were Persians forced into accepting Islam? Did Islam borrow its Hadith & Quran Teachings from Zoroastrianism & Persian?
Some articles online, such as this Iranica Encyclopedia article, claim that hadith were influenced by Iranian ideas and practices. Such as Prophet’s prohibition on being despondent and gloomy at someone’s loss, the Prophet’s prohibition on urinating while standing, encouraging the use of sewak or meswak which may have originated from sawaag a Persian word for the same thing. The usage of odd numbers such as 3, 7 or 33 when mentioning the amount of dhikr, the number of rakaats (prayer rounds) or multiplying of deeds etc. The Prophet’s mount for Mir’aj called Buraq may be Arabicized form of baraag or baarag “a riding beast, mount” in Persian language, and his Mi’raj is the similar experience as recorded in Book of Arda Wiraz by a Persian Prophet or religious figure named Wiraz. It is also claimed that the Prophet ﷺ must have adopted many traditions from his Persian companion named Salman Al Farsi (the Persian).
Prophet Muhammad ﷺ never prohibited being gloomy, what he prohibited was wailing, a pre-Islamic ignorant act among Arabs was wailing and beating when in grief: “beware of the devil’s croaking! As long as it comes from the eye and the heart, it is coming from mercy; and as long as it comes from the tongue and the hand, it is coming from Satan.” (Narrated from Ibn `Abbas by al-Bayhaqi in al-Sunan al-Kubra) The Prophet ﷺ prohibited his Ummah to urinate while standing but he also urinated in a very unclean place standing which made sense to avoid dirtying his clothes. (Sahih al-Bukhari 2471) The meswak or sewak, tooth stick introduced by the Prophet ﷺ cannot be the Persian use of sawag because the use of meswak led towards a victory of Muslims in the battle against the Persian army. The book, Healing Body & Soul by Amira Ayad tells that Saad bin Abi Waqas (r.a), the army general and victor against Persian battles noticed that the Muslims were being defeated continuously. After evaluation he concluded that the Muslims had forgotten the Sunnah of meswak. He immediately ordered the Muslims to use meswak and ask Allah ﷻ for forgiveness. The Persian spies in the Muslim camps saw the entire army using meswak thought that they were sharpening their teeth for cannibalism which spread fear among the Persians and they were routed. This shows meswak may not have been a Persian tradition. As for the usage of odd numbers like 3, 7 or 33, the Prophet Muhammad ﷺ stressed this fact because the Quran stresses on this fact: (Al Quran 89:1) “By the dawn and the even and the odd.” The even is the creation and the odd is Allah ﷻ because every even number is divisible such as 2, 4, 6, 8, but every odd is indivisible such as 1, 3, 5, 7, the even is creation that has pairs and opposites while Allah ﷻ is unique and without pairs. This is not a Persian tradition I believe, its a philosophy of the Quran about Allah ﷻ and His creation. As for barag in Persian that means “a riding beast or mount” cannot be equivalent to Buraq in Arabic because this term was not in use among Arabs for any riding beast. Prophet Muhammad ﷺ used this term for the supernatural conveyance. Buraq was a special animal of a special kind because buraq is derivative of root barq which means “intense” and “bright lightning” while Persian term does not carry the same meaning. On the other hand the Persian literature called Book of Arda Wiraz where Wiraz, supposedly a Persian Prophet, gave details of his dream journey where he saw insight in to hell, paradise and communicated with Ahura Mazda (Allah). The book is actually compiled in 9th and 10th Century CE during Islamic period, hence it could possibly have been influenced by traditions of Mir’aj. But even if this dream journey was authentic and pre-Islamic then Prophet’s of Allah ﷻ have received divine insight in to hell, paradise, day of judgment etc. But if one notices the details of the Mir’aj they conform to a physical journey experience than a dream, Prophet Muhammad ﷺ had never traveled to Jerusalem, yet, to the amazement of disbelievers, gave the correct details of city, traveling caravans and places in between. In fact even in our times many recent scientific & astronomical findings explain the Prophet’s Mir’aj as well, please read the following writing:
The Night Journey & Ascension - When Allah ﷻ Tested the Believers & the Disbelievers
Did Prophet Muhammad ﷺ Ever Travel To Jerusalem Before The Night Journey?
Scientific Viewpoint of the Night Journey & Ascension.
Additionally there may be some misconception among certain Western academia, or others who hold the belief that the Salaah (5 times prayers) comes from pre-Islamic Persian tradition. What many forget is that initially prayers in Islam were 50 and were reduced to 5. At one point they were only 2, then increased to 5. As for Salman Farsi (r.a), he converted to Islam at the blessed hands of Prophet Muhammad ﷺ during his prophet hood in the city of Madinah. Salman (r.a) was not present during first 13 years of the Prophet's mission in Makkah and this can be confirmed from all historical sources that mention the story of Salman (r.a) such as Salman the Persian. In fact during the Prophet's mission in Makkah there was no Persians that resided in the town and there was no influence of Zoroastrianism whatsoever.
1. HADITH v. AS INFLUENCED BY IRANIAN IDEAS AND PRACTICES - https://iranicaonline.org/articles/hadith-v
2. Ayad, Amira, and Jamila Hakam. Healing Body & Soul: Your Guide to Holistic Wellbeing following Islamic Teachings. pgs. 443-44
4. Salman The Persian - https://en.wikipedia.org/wiki/Salman_the_Persian
5. Pic taken from https://www.ames.cam.ac.uk/12th-century-fatimid-tiraz