Hadith 4:357
9223. 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 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 Messenger of Allah (ﷺ) about it. He said to him, '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 a people repented, and in it others will be forgiven.'"
حدثنا أبو معاوية عن عبد الرحمن بن إسحاق عن النعمان بن سعد قال أتى عليا رجل فقال يا أمير المؤمنين أخبرني بشهر أصومه بعد رمضان فقال لقد سالتني عن شيء ما سمعت أحدا يسأل عنه بعد رجل سمعته يسأل عنه رسول الله صلى الله عليه وسلم فقال له إن كنت صائما شهرا بعد رمضان فصم المحرم فإنه شهر الله وفيه يوم تاب فيه قوم ويتاب فيه على آخرين