There is a number of hadiths that have mentioned the fast of ‘Ashura:
With recourse to Shi‘ah sources of hadiths, we come to know that it has been narrated in some hadiths that the one who quits this fast has to atone and expiate for his actions for up to one year. It has also been reported that the Holy Prophet (S) himself used to fast on the day of ‘Ashura, and strongly advised everyone, even children, to fast on that day.
This demonstrates and proves that the day of ‘Ashura and its fast are overflowing with heavenly blessings.16
In other reported hadiths, the opposite has been narrated; that is to say, the fasting on the day of ‘Ashura has been forbidden and is one of the prohibited actions. Some other hadiths say it is an act of innovation, and fasting on that day is not a part of the religion.
Others have gone so far as to say that the reward of fasting on the day of ‘Ashura is the fire of Hell. It has been reported in other hadiths that the Holy Prophet never used to fast on that day.
As regards the conduct of the Infallible Imams (as), it is important to mention that no hadith has reached us suggesting that they or their companions used to fast on the day of ‘Ashura. If fasting on this day were mustahabb, the Infallibles would not fail to fast on that day.17
The hadiths which have been recorded in Sunni books regarding this issue are also varied. The meaning of many of these hadiths is that it is highly recommended [mustahabb-e mu’akkad] to observe the fast of ‘Ashura.
However, another group of hadiths contradict the first, in the sense that they say that the Holy Prophet (S) never used to fast on that day and never at all ordered anyone to fast on that day after the Qur’anic verse enacting the fasting of the month of Ramadan.18
16. Tahdhib al-Ahkam, vol. 4, pp. 299 -300, hadith 895, 906-908; Wasa’il al-Shi‘ah, vol. 10, section [bab] 20.
17. Wasa’il al-Shi‘ah, vol. 10, section [bab] 21; Al-Kafi, vol. 4, p. 146, hadith 4-7.
18. Sahih Bukhari, vol. 1, p. 341.
Adapted from: "The Uprising of Ashura and Responses to Doubts" by: "‘Ali Asghar Ridwani"