Musnad Ahmad 5:753

Abdullah narrated to us, Muhammad ibn al-Minhal, the brother of Hajjaj, narrated to us, Abd al-Wahid ibn Ziyad narrated to us, Abd al-Rahman ibn Ishaq narrated to us, from al-Nu’man ibn Sa’d, who said: A man said to Ali: "O Commander of the Faithful, which month do you command me to fast after Ramadan?" He said: "I have not heard anyone ask about this, except for a man who asked the Messenger of Allah, peace and blessings of Allah be upon him, and said: 'O Messenger of Allah, which month do you command me to fast after Ramadan?'" He said: "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 He forgave a people, and He will forgive a people in it."