Nu’man bin Sa’d said: A man came to Sayyiduna Ali (may Allah be pleased with him) and asked which month after Ramadan he should fast in. Sayyiduna Ali (may Allah be pleased with him) replied: Since I heard this from the Messenger of Allah (peace and blessings be upon him), no one has asked me about it until now. A man asked the Noble Prophet (peace and blessings be upon him) in which month of the year after Ramadan he should fast, so he (peace and blessings be upon him) instructed him to fast in Muharram, and said:“On this day Allah Almighty showed mercy to a people, and in the future He will show mercy to another people.”(The first people referred to are the Children of Israel, and the second people are certainly the Ummah of Muhammad, peace and blessings be upon him.)

It is narrated from Sayyiduna Abu Hurairah, may Allah be pleased with him, that the Noble Prophet, peace and blessings be upon him, said: "After Ramadan, the best fasts are those of the month which you call Muharram."

With this chain also, the aforementioned hadith is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him).