It was narrated that Ali (رضي الله عنه) said:
A man came to the Prophet (ﷺ) and said O Messenger of Allah, tell me of a month that I can fast after Ramadan. The Messenger of Allah (ﷺ) said: “If you want to fast a 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.`