Musnad Abu Ya'la al-Mawsili 3:167

Surayj ibn Yunus narrated to us, Abu Mu'awiyah narrated to us, 'Abd al-Rahman ibn Ishaq narrated to us, from al-Nu'man ibn Sa'd, from 'Ali, who said: A man came to the Prophet, may Allah bless him and grant him peace, and said: "O Messenger of Allah, inform me of a month that I should fast after Ramadan." He said: So the Messenger of Allah, may Allah bless him and grant him peace, said: "If you are to fast a month after Ramadan, then fast al-Muharram, for it is the month of Allah. In it is a day in which Allah forgave a people, and in it He will forgive others."