Fiqh-us-Sunnah - 3.125

Hadith 3.125.1

Fasting during the month of Muharram, especially the day of 'Ashurah and the days immediately preceding and following it

Abu Hurairah reported: "I asked the Prophet: 'Which prayer is the best after the obligatory prayers?' He said: 'Prayer during the middle of the night.' I asked: 'Which fast is the best after the fast of Rarnadan?' He said, 'The month of Allah that you call Muharram.' " This is related by Ahmad, Muslim, and Abu Dawud.
Mu'awiyyah ibn Abu Sufyan reported that he heard the Messenger of Allah say: "Concerning the day of 'Ashurah, it is not obligatory upon you to fast on it as I do.
Whoever wishes may fast and whoever does not wish to is not obliged to do so." This is related by al-Bukhari and Muslim.
'Aishah stated: "The tribe of Quraish used to fast on the day of 'Ashurah in the days before Islam, as did the Prophet. When he came to Madinah, he still fasted on it and ordered the people to do likewise. Then, when fasting during the month of Ramadan becam obligatory, he said: 'Whoever wishes may fast ['Ashurah] and whoever wishes may leave it." This is related by al-Bukhari and Muslim.
Ibn 'Abbas reported: "The Prophet came to Madinah and found the Jews fasting on the day of 'Ashurah. He said to them: 'What is this fast?' They said: 'A great day. Allah saved Moses and the tribes of Israel from their enemies on this day and therefore, Moses fasted on this day.' The Prophet said: 'We have more of a right to Moses than you,' so he fasted on that day also and ordered the people to fast on that day." This is recorded by al-Bukhari and Muslim.
According to al-Bukhari and Muslim, Musa al-Ash'ari reported: "The Jews would honor the day of 'Ashurah as an 'id. The Prophet said: 'You [Muslims] are to fast on it.'" Ibn 'Abbas reported: "The Messenger of Allah fasted on the day of 'Ashurah and ordered the people to fast on it.
The people said: 'O Messenger of Allah, it is a day that the Jews and Christians honor.' The Prophet said, 'When the following year comes--Allah willing--we shall fast on the ninth.' The death of the Prophet came before the following year." This is recorded by Muslim and Abu Dawud. In one version the wording is: "If I remain until next year, we shall fast the ninth," meaning, the tenth.
This is related by Muslim and Abu Dawud.
The scholars have mentioned that the fast of 'Ashurah is of three levels: fasting three days--that is, on the 9th, 10th, and 11th of Muharram; fasting on the 9th and 10th; and fasting only on the 10th.