The Messenger of Allah ﷺ used to say, "The best prayer after the obligatory prayers is the prayer in the depth of the night, and the best fasting after Ramadan is in the month of Allah which you call Muharram."
كان رسول الله ﷺ يقول: إن أفضل الصلاة بعد المفروضة الصلاة في جوف الليل، وأفضل الصيام بعد رمضان شهر الله الذي تدعونه المحرم.
Tabarani narrated it in Kabir (2/182-183), Nasai in Kubra (2904), and Bayhaqi (4/291), all of them from the hadith of Abdullah bin Amr, from Abdul-Malik bin Umayr, from Jundab bin Sufyan, and he related it. Nasai abridged it. Munziri said in Targhib wal-Tarhib (1548), after attributing it to Nasai and Tabarani, "Its isnad is sahih." Haythami said in Majma (3/190-191), "He attributed it in Atraf to Nasai. I did not find it in my copy, and it seems to be in Kubra. Tabarani narrated it in Kabir, and its narrators are the narrators of the sahih collections."On this topic there is also what was narrated from Mujeeba Bahiliyyah, from her father, or her uncle, that he came to the Prophet ﷺ, then left, and came back to him after a year, his condition and appearance having changed. He said, "O Messenger of Allah, do you not recognize me?" He said, "Who are you?" He said, "I am the Bahili who came to you last year." He said, "What has changed you, when you used to have a good appearance?" He said, "I have not eaten food since I parted from you except at night!" The Messenger of Allah ﷺ said, "Why have you tormented yourself?" Then he said, "Fast the month of patience, and one day from every month." He said, "Give me more, for I have strength." He said, "Fast two days." He said, "Give me more." He said, "Fast three days." He said, "Give me more." He said, "Fast from the sacred months and leave off, fast from the sacred months and leave off, fast from the sacred months and leave off," and he gestured with his three fingers, closing them and then releasing them.Abu Dawud narrated it (2428) via Hammad, Ibn Majah (1741), and Nasai (2743) from the hadith of Sufyan, and Imam Ahmad (20323) from the hadith of Jareeri, all of them from Abu Sabeel, from Mujeeba Bahiliyyah, and he related it. This is how Abu Dawud narrated it, and the wording is his. Ibn Majah narrated it, saying, "from Abu Mujeeba Bahili, from his father, or from his uncle." In it, after his saying "two days," "Fast the month of patience, and three days after it, and fast the sacred months." In Nasai's version, "from Mujeeba Bahili, from his uncle," without doubt, and in it, "Fast the sacred months and break the fast." In Ahmad's version, "Mujeeba, an old woman from Bahila." Mujeeba Bahiliyyah is unknown, no one narrated from her except Abu Sabeel. Then, as you have seen, people disagreed about her, so it was said that she was a woman, and it was said that she was a man, and it was said "Abu Mujeeba."Munziri said, after quoting the scholars' statements about Mujeeba Bahiliyyah or Bahili, "This disagreement has occurred as you see, and some of our teachers indicated that it should be graded daif for that reason, and this is well founded."Jareeri is Saeed bin Iyas, who became confused toward the end of his life, but some of those mentioned above narrated from him before the confusion set in. His saying "the sacred months" means the sacred months, which are Rajab, Dhul-Qaidah, Dhul-Hijjah, and Muharram. A Bedouin expressed this by saying, "three in a row, and one alone."On this topic there is also what was narrated from Ali bin Abu Talib, that a man asked him, "Which month do you order me to fast after the month of Ramadan?" He said to him, "I have not heard anyone ask about this except a man whom I heard ask the Messenger of Allah ﷺ, while I was sitting, 'O Messenger of Allah, which month do you order me to fast after the month of Ramadan?' He said, 'If you are to fast after the month of Ramadan, then fast Muharram, for it is the month of Allah. In it there is a day on which He accepted the repentance of a people, and in which He will accept the repentance of another people.'"Tirmidhi narrated it (741) from Ali bin Hajar, Ali bin Mushir informed us, from Abdul-Rahman bin Ishaq, from Numan bin Saad, from Ali bin Abu Talib, and he related it. Tirmidhi said, "Hasan, singular." I say, it is not hasan, for it contains Abdul-Rahman bin Ishaq, who is Abu Shayba Wasiti, daif by agreement of the scholars. Numan bin Saad was deemed reliable by no one except Ibn Hibban, which is why Hafiz said of him, "Acceptable," meaning if corroborated, but I did not find anyone who corroborated him. Rather, Tirmidhi confirmed that it is singular.On this topic there is also what was narrated from Ibn Abbas, who said, the Messenger of Allah ﷺ said, "Whoever fasts the day of Arafah, it will be an expiation for two years, and whoever fasts a day of Muharram, he will have thirty days for every day."Tabarani narrated it in Saghir (963), from Muhammad bin Razin bin Jamee Misri, Abu Abdullah Madani, Haytham bin Habib told us, Sallam Taweel told us, from Hamza Zayyat, from Layth bin Abu Salim, from Mujahid, from Ibn Abbas, and he related it. Tabarani said, "No one narrated it from Hamza Zayyat except Sallam Taweel, and Haytham bin Habib narrated it alone." Haythami said in Majma (3/190), "It contains Haytham bin Habib, whom Dhahabi graded daif." I say, his teacher Sallam Taweel is "discarded," as stated in Taqrib. It also contains Layth bin Abu Salim, who became confused toward the end of his life, and his hadith could not be distinguished from the rest, so it was discarded.So Munziri's statement in Targhib wal-Tarhib (1549), "Tabarani narrated it in Saghir, and it is singular, and its isnad is not problematic, and Ibn Hibban deemed Haytham bin Habib reliable," how can it be "not problematic," when it contains all that I have mentioned? Moreover, I did not find Ibn Hibban's endorsement of Haytham bin Habib in Thiqat.On this topic there is also a narration from Anas bin Malik. Tabarani narrated it in Awsat, and it contains Yaqub bin Musa Madani, who is unknown, and it also contains Maslama bin Rashid, of whom Abu Hatim said, "His hadith is disordered." See Majma (3/191).