What is mentioned on fasting Muharram and the sacred months

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.'"

Al-Hasan used to fast in the sacred months.

Sayyiduna Ibn Umar (may Allah be pleased with them both) used to fast in Makkah during the sacred months.

Sayyiduna Abu Hurayrah (may Allah be pleased with him) said: A man came to the Messenger of Allah (peace and blessings be upon him) and said: "O Messenger of Allah! Which fasting is the most virtuous after Ramadan?" He said: "The fasting of the month of Allah which you call Muharram."