A man said to Ali, "Commander of the Believers, which month do you advise me to fast after Ramadan?"Ali replied, "I have not heard anyone ask about this since a man asked the Messenger of Allah ﷺ, saying, 'Messenger of Allah, which month do you advise me to fast after Ramadan?' He answered, 'If you wish to fast a month after Ramadan, then fast Muharram, for it is the month of Allah. In it there is a day on which He accepted the repentance of one people, and in it He will accept the repentance of another people.'"
حدثنا عبد الله حدثنا محمد بن المنهال أخو حجاج حدثنا عبد الواحد بن زياد حدثنا عبد الرحمن بن إسحاق
قال رجل لعلي يا أمير المؤمنين أي شهر تأمرني أن أصوم بعد رمضان فقال ما سمعت أحدا سأل عن هذا بعد رجل سأل رسول الله ﷺ فقال يا رسول الله أي شهر تأمرني أن أصوم بعد رمضان فقال إن كنت صائما شهرا بعد رمضان فصم المحرم فإنه شهر الله وفيه يوم تاب على قوم ويتوب فيه على قوم
Its isnad is daif, due to the weakness of Abdul-Rahman bin Ishaq. Tirmidhi related the hadith, 2:53-54, through Abdul-Rahman bin Ishaq, and said, "hasan gharib hadith." His commentator said that Nasai also related it, and that Ibn Hibban, Ibn Abdul-Barr, and Ibn Hazm graded it sahih, as stated in Umdat Qari. The merit of fasting the month of Muharram has been established as sahih through the hadith of Abu Hurairah, see Muntaqa 1235.