Fasting in the month of Muharram is highly desired, especially on the tenth day, ' Ashura. The Messenger of Allah (saas) has been reported as saying when asked which prayer is better after obligatory prayer: "Prayer in the middle of the night." He was asked again which fast is better after Ramadan. He said: "In the month that is called Al-Muharram." (Ahmed, Muslim, and Abu Dawud)
This hadith indicates the importance of voluntary fasting in Muharram, among all the months. The Prophet (saas) however, emphasizes the fast of the tenth day, 'Ashura. The word "`Ashura" is derived from `Ashara, which means ten. The observation of this day goes back to Prophet Musa bin 'Emran (saas). In a hadith related by Ibn Abbas (raa) when the Messenger of Allah (saas) came to Medina, he found the Jews fasting the day of 'Ashura. He inquired why they did so. They replied that it was a good day, the day which Allah delivered Prophet Musa (saas) and the children of Israel from their enemy. As gratitude Musa (saas) fasted that day.
The Prophet (saas) responded: "I have more right to Musa than you." He fasted the day and commanded the believers to fast." (Agreed upon.)
This hadith indicates that the Prophet (saas) was in complete agreement with Prophet Musa (saas) as well as the other prophets. The point here is that the Messenger of Allah would always do an act of worship if it was a prophecy reported from any of the prophets. Earlier, we see how he told us that the best voluntary fast is the fast of Dawud.
This is why the following hadith is of great interest to us. Ibn Abbas related when Prophet Muhammad (saas) observed the day of ' Ashura and commanded his followers to observe it, they asked him: "O Messenger of Allah (saas) this is the day the Jews, and Christians respect and honor..." The Prophet (saas) promised them that "Next year Allah willing, we shall fast the ninth, tasuu'aa, along with the tenth." By the next Muharram, the Prophet (saas) had already passed away. Because the believers, desire to emulate Prophet Musa in fasting on ' Ashura do so with the desire not to participate in the festivity of the People of the Book, who have reduced the day to a mere formality. The Prophet (saas) decided to remove this ambiguity with the fast of the ninth day.
Our 'Ulama, analyzing all reports from the Prophet (saas) regarding Muharram, stated the fast of Muharram could be viewed in three ways:
1. Fasting three days of Muharram, the ninth, tenth and eleventh.
2. Fasting on the ninth and tenth days because of two previous Hadiths.
3. Fasting on the tenth day alone.
It is to celebrate on the day of `Ashura. In a hadith by Jabir Bin Abdullah, the Messenger of Allah (saas) said: "Whoever spends generously on himself and his family on the day of ` Ashura, Allah will provide for him generously the rest of his year." (Al-Bayhaqi)