Did you observe any fast in the middle of this month (Sha'ban)? He said: No. Thereupon the Messenger of Allah ﷺ said: Fast for two days instead of (one fast) when you have completed (fasts of) Ramadan.

أَنَّ النَّبِيَّ ﷺ قَالَ لِرَجُلٍ هَلْ صُمْتَ مِنْ سُرَرِ هَذَا الشَّهْرِ شَيْئًا قَالَ لاَ فَقَالَ رَسُولُ اللَّهِ ﷺ فَإِذَا أَفْطَرْتَ مِنْ رَمَضَانَ فَصُمْ يَوْمَيْنِ مَكَانَهُ