"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)
"إِذَا صُمْتَ منَ الشَّهرِ ثلاثًا فَصُمْ ثلاثَ عَشْرَةَ وأرْبَعَ عَشْرَةَ وخَمْسَ عْشرةَ" .