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. (Using translation from Abū Dāʾūd 2328)

أَنَّ رَسُولَ اللهِ ﷺ قَالَ لَهُ أوَ لِرَجُلٍ أَصُمْتَ مِنْ سَرَرِ شَعْبَانَ شَيْئًا قَالَ لَا قَالَ فَإِذَا أَفْطَرْتَ فَصُمْ يَوْمَيْنِ