Musannaf Ibn Abi Shaybah 9470 — What is mentioned on fasting Muharram

Al-Nu'man bin Sa'd said: A man came to Sayyiduna Ali (may Allah be pleased with him) and said: "O Commander of the Faithful! In which month should I fast after Ramadan?" Sayyiduna Ali (may Allah be pleased with him) said: "You have asked me about something I have not heard anyone ask about since a man whom I heard asking the Messenger of Allah (peace and blessings be upon him) about it. He said to him: 'If you are going to fast a month after Ramadan, then fast the month of Muharram, for it is the month of Allah. In it there is a day on which Allah accepted the repentance of a people, and on which He will accept the repentance of others.'"