Abdullah bin al-'Ala' عبد الله بن العلاء بن زبر · Fasting (Kitab Al-Siyam)

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.