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. (Using translation from Muslim 1161c)

أَنَّ النَّبِيَّ ﷺ قَالَ لِرَجُلٍ هَلْ صُمْتَ مِنْ سِرَارِ هَذَا الشَّهْرِ شَيْئًا؟ فَقَالَ لَا فَقَالَ رَسُولُ اللهِ ﷺ فَإِذَا أَفْطَرْتَ مِنْ رَمَضَانَ فَصُمْ يَوْمَيْنِ مَكَانَهُ