Musnad Abu Ya'la al-Mawsili 3:166

Abu Bakr ibn Abi Shaybah narrated to us, Abu Mu'awiyah narrated to us, from Abd al-Rahman ibn Ishaq, from al-Nu'man ibn Sa'd, who said: A man came to Ali and said, "O Commander of the Faithful, inform me of a month that I should fast after Ramadan." He said: "You have asked me about something that I have not heard anyone ask about, except for a man I heard asking the Prophet, peace and blessings of Allah be upon him. He said to him: 'If you are to fast a month after Ramadan, then fast al-Muharram, for it is the month of Allah, in which there is a day on which Allah forgave a people, and He will forgive others on it.'"