Original answer written on Dec 30, 2019 - Is fasting on 10th of Muharram, the day of Ashura a fabrication in Islam?
Prophet Muhammad ﷺ ordered the Muslims to fast this day after he saw the Jews fasting this day as a celebration because Allah ﷻ saved them from Pharaoh on this day. Hence he said: “I am closer to Moses than they (Jews)." So, he observed the fast (on that day) and ordered the Muslims to fast on it.” (Sahih al-Bukhari 3397)
Unfortunately some circles among Shia Muslims question the credibility of this hadith of fasting on Ashura because they allege that Prophet’s beloved grandson Imam Hussain (alaihi salaam) was killed on this day hence the Bannu Umayyad Caliphs shifted the focus of this crime by introducing this hadith among the Muslims so that it is considered a day of celebration. To support this claim they give the following reasons for questioning its credibility:
Firstly the hadith narration is not only limited to four companions, in fact it has also narrated by an Ansari (Helper) companion named Abu Qatada Al Ansari (r.a) who was an older companion when the Prophet ﷺ was in Madinah and the hadith is found in Sahih Muslim:
Abu Qatada al-Ansari (Allah be pleased with him) reported that the Messenger of Allah (ﷺ) was asked about his fasting: “He was asked about fasting on the day of 'Ashura (10th of Muharram), whereupon he said: It expiates the sins of the preceding year.” (Sahih Muslim 1162 b)
Additionally, it was also narrated by the most intimate and beloved wife of the Prophet, Ayesha (r.a) who knew all his teachings and practices. As for the four companions Ibn Abbas (r.a), Abu Musa Al-Ashari (r.a), Abu Huraira (r.a) and Muawiya may not have been in Madinah or were very young yet that does not nullify the fact that they were aware of what the practices of the Prophet and his companions in Madinah used to be. They were not very little at the time of narrating this incident, they were reputable scholars in Madinah, Kufa and Makkah where they were teaching traditions to other Muslims. Also the fasting of Ashura was made optional (nafl) once Ramadaan became obligatory (fard) in Madinah but before Ramadaan became obligatory (fard) the Ashura fast was of obligatory nature. Hence all Muslims in Madinah strongly adhered to this act of worship. Such as Ibn Umar (r.a) said: “But when (the order of compulsory fasting) in the month of Ramadan was revealed, the Prophet said, "It is up to one to fast on it (i.e. day of 'Ashura') or not." (Sahih Al Bukhari 4501)
Answer to the second point
As for the term “Ashura” it did exist even in the Jewish tradition. The word "tenth" in Hebrew is Asarah or Asharah (עשרה), which is from the same Semitic root A-SH-R. For instance the in the Torah Book of Leviticus designates the tenth day of the seventh month as holy and a fast (Lev. 16, Lev. 23, Num. 29) and the Jewish term used for the 10th day is Asharah (עשרה). Leviticus 16 / Hebrew. Most importantly the Quraysh used to observe fast in Makkah on this day even in pre-Islamic times as Narrated Aisha (r.a) : “'Ashura' (i.e. the tenth of Muharram) was a day on which the tribe of Quraish used to fast in the prelslamic period of ignorance.” (Sahih Al Bukhari - 3831)
Answer to the third point
The modern Jews don’t fast on this day which is correct. We also need to keep in mind that Jews, all of them do not follow same thought, there are various sects and groups that have existed in the past and still exist. Some have survived down to our times while others perished in history. For instance the Quran claims that Jews worship Uzair (Ezra) as the son of God but modern Jews don’t follow that practice but the Jews of Madinah did follow this belief and hence they were a separate group that carried that belief: (Al Quran 9:30) AND THE JEWS say, "Ezra is God's son," while the Christians say, "The Christ is God's son." Such are the sayings which they utter with their mouths, following in spirit assertions made in earlier times by people who denied the truth!" [They deserve the imprecation:] "May God destroy them!" - Whats more interesting to note is that the day of Ashura usually coincides with the 10th day of Tishri one of the months of Hebrew Calendar. Tishri usually falls in September - October of the global Gregorian Calendar and in the year of 2018 both Ashura and tenth of Tishri fell on the exact same day. This year (2019) the 10th Ashura was on September 10th and 10th Tishri was on September 11th. The tenth day of Tishri is the Jewish holiday of Yom Kippur which is still a day when Jews fast! Even though Yom Kippur is celebrated because Prophet Musa (alaihi salaam) came back from Mt. Sinai. Nevertheless it does show some coincidence.
Answer to the fourth point
Prophet Muhammad ﷺ was already fasting himself before he moved to Madinah. After he moved to Madinah he ordered the Muslims to fast with him and made it obligatory (fard) for everyone as narrated Aisha (r.a): “The Prophet (ﷺ) also used to fast on this day. So when he migrated to Medina, he fasted on it and ordered (the Muslims) to fast on it.” (Sahih Al Bukhari 3831) This pre-Islamic tradition may have been because of the holiness of this day in the history of mankind such as the incidents of:
• Allah created the heavens, mountains, oceans, pen (qalam) and tablet (lawh) on this day.
• Hazrath Ibrahim (alaihi salaam) was born, and Allah ﷻ saved him from the furnace on this day.
• Pharaoh was drowned,
• Allah ﷻ forgave the transgression Adam (alaihi salaam) ,
• Isa (alaihi salaam) was born on this day
• The day of Reckoning will occur on this day.
(Al Ghunya Talibi Tariq al-Haqq )
The apparent wording of some ahadith claim that “he started fasting when he came to Madinah” doesn’t mean that his arrival happened in Muharram. It may have happened in some other month but the order to fasting was given to Muslims in the month of Muharram after his arrival in Madinah and thats what it means.
Answer to the fifth point
Prophet Muhammad ﷺ was not only following Jews in their fasting on the day of Ashura. In fact he followed also their Qiblah (Jerusalem) before Kabah (Makkah) was finalized as the Qiblah for Muslims. He also gave Shariah punishments in according to the Jewish law such as stoning as well as others practices. These he followed to prove that his revelation is not separate from what was taught by the previous prophets and their followers. Nevertheless he ordered the Muslims to fast an extra day with the 10th of Muharram (i.e. 9th or 11th), so that in this act Muslims are considered distinct from Jews and Judaism.