Musnad Ahmad 5:765

Abdullah narrated to us, Zuhayr Abu Khaythama narrated to me, Abu Mu'awiyah narrated to us, Abd al-Rahman ibn Ishaq narrated to us, from al-Nu'man ibn Sa'd, from Ali, may Allah be pleased with him, 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." 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, and in it is a day in which He forgave a people, and in it He will forgive others."