I’ve just got an informative forward regarding fasting in Muharram:
The Virtue of observing more nafil fasts during Muharram:
Abu Hurayrah (ra) said: “The Messenger of Allah (saws) said: ‘The best of fasting after Ramadan is fasting Allah’s month of Muharram.’” (reported by Muslim).
The phrase “Allah’s month”, connecting the name of the month to the name of Allah in a genitive grammatical structure, signifies the importance of the month. Al-Qari said: “The apparent meaning is all of the month of Muharram.” But it was proven that the Prophet (saws) never fasted any whole month apart from Ramadan, so this hadeeth is probably meant to encourage increasing one’s fasting during Muharram, without meaning that one should fast for the entire month.
It was reported that the Prophet (saws) used to fast more in Sha’ban. It is likely that the virtue of Muharram was not revealed to him until the end of his life, before he was able to fast during this month. (Sharh al-Nawawi ‘ala Saheeh Muslim).
It is mustahabb (encouraged) to fast Tasoo’a’ with ‘Ashoora’:
‘Abd-Allah ibn ‘Abbas (ra) said: “When the Messenger of Allah (saws) fasted on ‘Ashoora’ and commanded the Muslims to fast as well, they said, ‘O Messenger of Allah, it is a day that is venerated by the Jews and Christians.’ The Messenger of Allah (saws) said, ‘If I live to see the next year, in sha Allah, we will fast on the ninth day too.’ But it so happened that the Messenger of Allah (saws) passed away before the next year came.” (Reported by Muslim).
Al-Shafa’i and his companions, Ahmad, Ishaq and others said: “It is mustahabb to fast on both the ninth and tenth days, because the Prophet(saws) fasted on the tenth, and intended to fast on the ninth.”
On this basis it may be said that there are varying degrees of fasting ‘Ashoora’, the least of which is to fast only on the tenth and the best of which is to fast the ninth as well. The more one fasts in Muharram, the better it is.