مسند أحمد

It was narrated that an-Nu’man bin Sa’d said: A man said to ‘Ali (رضي الله عنه): O Ameer al-Mu`mineen, what month do you advise me to fast after Ramadan? He said: I have never heard anyone ask about this after a man asked the Messenger of Allah (ﷺ) : O Messenger of Allah, what month do you advise me to fast after Ramadan? He said: “If you want to fast any month after Ramadan, then fast al-Muharram, for it is the month of Allah and in it is a day on which Allah forgave people and will forgive (more) people.`