Sunan At-Tirmidhi

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