Nu’man bin Sa’d says that a man asked Sayyiduna Ali (may Allah be pleased with him): O Commander of the Faithful! After Ramadan, which month’s fasting do you emphasize for me to observe? He replied: I heard only one man ask the Prophet (peace and blessings be upon him) such a question, and after him, you are the only person from whom I am hearing this question. In response, the Prophet (peace and blessings be upon him) said: “If you want to fast in any month after Ramadan, then fast in the month of Muharram, for it is Allah’s month. In it is a day on which Allah accepted the repentance of a people, and Allah will accept the repentance of a people (again).”
Hadith Referenceمسند احمد / مسند الخلفاء الراشدين / 1322
Hadith Gradingحکم دارالسلام:إسناد ضعيف لضعف عبدالرحمن بن إسحاق الواسطي و جهالة النعمان بن سعد. وفي مسلم : 1163، عن ابي هريرة مرفوعاً : «افضل الصيام بعد رمضان شهر الله المحرم.»