Imran reported from the Prophet ﷺ that he asked him, or asked a man while Imran was listening, "So-and-so, did you not fast during the last days of this month?" The narrator said, I think he meant Ramadan.The man replied, "No, Messenger of Allah."He said, "Then when you break your fast, fast two days."
عن النبي ﷺ أنه سأله أو سأل رجلا وعمران يسمع، فقال: يا فلان، أما صمت سرر هذا الشهر؟ قال: أظنه قال يعني رمضان، قال الرجل: لا، يا رسول الله. قال: فإذا أفطرت فصم يومين.
Bukhari narrated it in the Book of Fasting (1983), and Muslim in the Book of Fasting (1161), via Mahdi bin Maymun, who said, "Ghaylan bin Jareer told us, from Mutarrif, from Imran bin Husayn," and he related it. The wording given is Bukhari's. His saying "Ramadan" is a mistake, and the correct word is Shaban, as Bukhari himself pointed out, mentioning it from Thabit as a suspended narration, while in Muslim it is connected. Muslim's wording is, "O so-and-so, have you fasted any of the last part of this month?"In another report found in Muslim (199), via a different route, from Hammad bin Salama, from Thabit, from Mutarrif, from Imran bin Husayn, that the Messenger of Allah ﷺ said to him, or to another man, "Have you fasted any of the last part of Shaban?" He said, "No." He said, "Then when you break your fast, fast two days." Shuba had no doubt that it was he himself, not that it was said to a different man.He narrated it from the nephew of Mutarrif bin Shakhir, who said, "I heard Mutarrif narrate from Imran bin Husayn that the Messenger of Allah ﷺ said to a man, 'Have you fasted any of the last part of this month?'" meaning Shaban. He said, "No." So he said to him, "When you finish Ramadan, fast one day or two days," Shuba being uncertain about which. "And I think he said, 'two days.'" Ahmad (19839) narrated it from Muhammad bin Jafar, who said, "Shuba told us," and he related it.Muslim narrated it through two routes (Muhammad bin Jafar and Nadr bin Shumayl), both from Shuba. In this narration, "this month" is specified as being Shaban.His use of the word "sarar" (with two fatha vowels) means its last part.What Abu Dawud narrated (2330) via Uzai, that he said, "Its sirr is its beginning," is a mistake, as Khattabi said. He said, "The correct view is that its sirr is its end. This is how our companions related to us, from Ishaq bin Ibrahim bin Ismail, who said, 'Mahmud bin Khalid Dimashqi told us, from Walid, from Uzai, who said, "Its sirr is its end."' This is the correct view. There are several dialectal forms, one says 'sirr of the month,' 'sarar of the month,' and 'sararah,' and the last part of the month is called sirr because the moon is hidden during it." End of quote.This is also what Bayhaqi confirmed as correct (4/211), where he said, "He meant by it the day or two days during which the moon is hidden before the day of doubt, and he meant fasting the last part of the month along with the day of doubt, if that coincided with his habit of fasting the last day of every month. It has also been said that he meant by sirr its middle, since the sirr of anything is its interior, and according to this he meant the White Days." End of quote.It has also been said that perhaps the reason for this was that he was accustomed to fasting its last part, or had vowed to do so, and then left it off because of the apparent prohibition against anticipating Ramadan by a day or two, so it was clarified that what is habitual or vowed is not covered by that prohibition.There are other interpretations of this hadith mentioned by Hafiz in Fath al-Bari (4/231-232), so refer to it. The interpretation I find most satisfactory among these is the one I have already mentioned, that the meaning of sirr is its last part, and what is prohibited is anticipating Ramadan by a day or two without it being habitual or vowed. Perhaps this was before the fasting of Ramadan was made obligatory, and afterward it became something voluntary for whoever had a habit or a vow.Of similar meaning is what was reported from Muawiya, who said, "I heard the Messenger of Allah ﷺ say, 'Fast the month and its sirr.'" Abu Dawud narrated it (2329) from Ibrahim bin Ala Zubaydi, from his book, "Walid bin Muslim told us, Abdullah bin Ala told us, from Abu Azhar (Mughira bin Farwa), who said, 'Muawiya stood up among the people at Dayr Mishal, which is by the gate of Hims, and said, "O people, we saw the new moon on such and such a day, and I am going ahead with the fast, so whoever wishes to do the same may do so."'" He said, "Malik bin Hubayra Sabai then stood up to him and said, 'O Muawiya, is this something you heard from the Messenger of Allah ﷺ, or is it your own opinion?'" He said, "I heard the Messenger of Allah ﷺ say," and he related the hadith.Abu Azhar (Mughira bin Marwa Thaqafi), of Dimashq, is not well known in the pursuit of knowledge, and no one other than Ibn Hibban rated him reliable. Hence Hafiz said, "Acceptable," meaning if corroborated, otherwise the hadith is weak. Also in the chain is Walid bin Muslim, who practiced tadlis, specifically tadlis al-taswiyah, but he stated his source explicitly, and according to the majority it suffices that he state it explicitly at the beginning of the chain, though it would have been better had he stated it explicitly at every level.