It was narrated that Abu Hurayrah (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said:
“The best fasting after Ramadaan is the month of Allaah, Muharram, and the best prayer after the obligatory prayer is prayer at night.”
(Narrated by Muslim, 1163)
With regard to the phrase “the month of Allaah”, mentioning the month in conjunction with Allaah is a sign of its great status. Al-Qaari said: it seems that what is meant is all of the month of Muharram.
But it was proven that the Prophet (peace and blessings of Allaah be upon him) did not fast any month in full apart from Ramadaan, so this hadeeth is to be understood as meaning that it is it is encouraged to fast a lot in the month of Muharram, not to fast the whole month.