Numan ibn Sad reported from Sayyidina Ali that a man asked him, “In which month after Ramadan, do you command me to fast?” He said, “I did not hear anyone ask about it except a man whom I heard ask Alah’s Messenger (SAW) while I was sitting by him. So, he had asked, 0 Messenger of Allah! Which month, besides the month of Ramadan do you command me that I should fast?’ He had said, ‘If I were to fast besides the month of Ramadan then I would fast during Muharram, for, it is the month of Allah’ There is a day in it when Allah accepted the repentance of a people and, on it, will relent towards another people (too)”.
[Ahmed1321]