The Prophet(sallallahu alaiyhi wassallam) has said: “The best fasts after the fasts of Ramadhan are those of the month of Muharram.”
Although the fasts of the month of Muharram are not obligatory, yet, the one who fasts in these days out of his own will and choice is entitled to a great reward by Allah Almighty. The Hadith cited above signifies that the fasts of the month of Muharram are most rewardable ones among the Nafl fasts i.e. the fasts one observes out of his own choice without being obligatory on him.
The hadith does not mean that the award promised for fasts of Muharram can be achieved only by fasting for the whole month. On the contrary, each fast during this month has merit. Therefore, one should avail of this opportunity as much as he can.
Nabi(sallallahu alaiyhi wassallam) was asked with regard to the blessings of this fast, he replied:”
It is a compensation for the sins of the past year.” i.e. Allah will forgive the sins of the past year.”
(Muslim, Vol 1). Abu Qataada (radhiyallahu anhu) has related that the Prophet(sallallahu alaiyhi wassallam) has reported to have said, ” It is my thought that by fasting on the 10th of Muharram Allah Ta’ala will pardon the sins of the past year.” (Tirmizi). Note: For the forgiveness of major sins Taubah and Istighfaar is a pre-requisite.
Hadhrat Ibn Abbaas(radhiyallahu anhu) narrates that when Nabi(sallallahu alaiyhi wassallam) came to Madinah and found the Jews fasting on the day of Aashura he inquired, ” What is the significance of this day on which you fast? ” They replied, ” This is the day Allah saved Nabi Moosa(alaiyhis salaam) and his followers and drowned Fir’oun and his army. Moosa(alaiyhis salaam) fasted on this day as a token of thanks – giving, thus we too fast on this day.” Nabi (sallallahu alaiyhi wassallam) said: ” We are more worthy of Moosa(alaiyhis salaam) than you and we are more closer to him than you are.” Thereafter he fasted on this day and ordered the Sahaabah to do the same”. (Muslim, Vol 1)
According to another hadith, it is more advisable that the fast of ‘Ashurah should either be prefixed or suffixed by another fast. It means that one should fast two days: the 9th an 10th of Muharram or the 10th and 11th of it. The reason of this additional fast as mentioned by the Prophet (Sallallahu Alaihi Wassallam) is that the Jews used to fast on the day of Aashurah alone, and the Prophet (Sallallahu Alaihi Wassallam) wanted to distinguish the Islamic-way of fasting from that of Jews. Therefore, he advised the Muslims to add another fast to that of Aashurah.
Rasulullah(sallallahu alaiyhi wassallam) is reported to have said: ” Observe the fast of Aashura and oppose (the ways of) the Jews. Fast a day before it (also) or a day after.” Hence, it is better to fast on the 9th and 10th or on the 10th and 11th.