(40) باب مَا جَاءَ فِي صَوْمِ الْمُحَرَّمِ
An-Nu'man bin Sa'd narrated:
"A man asked Ali: "Which month do you order me to fast after the month of Ramadan?' He said to him, 'I have not heard anyone ask this except for a man whom I heard asking the Messenger of Allah while I was sitting with him. He said: "O Messenger of Allah! Which month do you order me to fast after the month of Ramadan?" He said: "If you will fast after the month of Ramadan, then fast Al-Muharram, for indeed it is Allah's month in which there is a day that Allah accepted the repentance of a people, and in which He accepts the repentance of other people."
Reference: Jami` at-Tirmidhi 741
In-book reference: Book 8, Hadith 60
English translation: Vol. 2, Book 3, Hadith 741