Every fast in the month of Muharram is rewarded manifold by Allah Ta’ala. It is reported by Ibn Abbas (R.A.) that Rasulullah (sallallahu alaihi wasallam) said: “He who fasts on the day of Arafat, his fast will be a compensation for the sins of two years and the one who keeps a fast in the month of Muharram will receive the reward of thirty fasts for each fast (in the sacred month) (At-Targheeb Wat Tarheeb - Vol.2, pg.114).
FASTING ON THE DAY OF ASHURA (10TH OF MUHARRAM)
The tenth of Muharram, known as the day of Ashoora, is a very important and significant day in the Islamic calendar. Rasulullah (sallallahu alaihi wasallam) has exhorted the Ummah to fast on this day. Some Ulama are of the opinion that before the fasts of Ramadhaan were prescribed, the fast of Ashoora was compulsory upon the Ummah. However, once the fast of Ramadhaan became obligatory, the compulsion of the fast of Ashoora was abrogated.
Once Rasulullah (Sallallahu Alaihi Wasallam) was asked regarding the virtue of fasting on the tenth of Muharram. He replied: “It is a compensation of the sins of the past year (Sahih Muslim).” However, together with fasting on the tenth of Muharram one should also try to fast a day before or after it as well. This is to avoid resembling the Jews who also fasted on the tenth of Muharram.