The month of Muharram is the first month of
the Arabic year, and it is one of the four sacred months of Allaah.
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.
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.