The Messenger of Allah (ﷺ) asked a man: Did you fast the last day of Sha'ban ? He replied: No. He said: If you did not observe a fast, you must fast for a day. One of the two narrators said: For two days.
Sunan Abi Dawud Book 14, Hadith 16
← Previous Hadith The Prophet (ﷺ) said: Do not fast one day or two days just before Ramadan except in the case of a man who has been in the habit or observing a fast (on that day); and do not fast until you sight it (the moon). Then fast until you sight it. If a cloud appears on that day (i.e. 29th of Ramadan) then complete the number thirty (days) and then end the fasting: a month consists of twenty-nine days.

Sunan Abi Dawud Book 14, Hadith 15 Next Hadith → AbulAzhar al-Mughirah ibn Farwah said: Mu'awiyah stood among the people at Dayr Mustahill lying at the gate of Hims. He said: O people, we sighted the moon on such-and-such day. We shall fast in advance. Anyone who likes to do so may do it. Malik ibn Hubayrah as-Saba'i stood up and asked: Mu'awiyah, did you hear the Messenger of Allah (ﷺ) say something (about this matter), or is this something on the basis of your opinion? He replied: I heard the Messenger of Allah (ﷺ) as saying: Fast the month (in the beginning) and in the last.

Sunan Abi Dawud Book 14, Hadith 17