"I heard Abu Dharr, at Ar-Rabadhah, saying: 'The Messenger of Allah said: if you want to fast at any time during the monthm then fast on the thirteenth, fourteenth and fifteenth."' (Using translation from Nasāʾī 2424)
أَنَّهُ قَالَ قَالَ لِي رَسُولُ اللهِ ﷺ إِذَا صُمْتَ مِنْ شَهْرٍ ثَلَاثًا فَصُمْ ثَلَاثَ عَشْرَةَ وَأَرْبَعَ عَشْرَةَ وَخَمْسَ عَشْرَةَ