Saeed Jareeri reported from Abu Ala, from Mutarrif, from Imran bin Husayn that the Messenger of Allah ﷺ said to a man, "Have you fasted any part of the month of Shaban?"The man replied, "No."The Prophet ﷺ said, "When you break your fast, then fast a day." One of the two narrators said, "two days."
حدثنا موسى بن إسمعيل حدثنا حماد عن ثابت عن مطرف
وسعيد الجريري عن أبي العلاء عن مطرف عن عمران بن حصين أن رسول الله ﷺ قال لرجل هل صمت من شهر شعبان شيئا قال لا قال فإذا أفطرت فصم يوما وقال أحدهما يومين
Muslim narrated it in the Book of Fasting, chapter "Fasting the Last Days of Shaban," hadith no. 1161b/199, from the hadith of Hammad bin Salama, and Bukhari narrated it in the Book of Fasting, chapter "Fasting at the End of the Month," hadith no. 1983, from the hadith of Mutarrif, with the same wording.