قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «يَا أَبَا ذَرٍّ إِذَا صُمْتَ مِنَ الشَّهْرِ ثَلَاثَةَ أَيَّامٍ فَصُمْ ثَلَاثَ عَشْرَةَ وَأَرْبَعَ عَشْرَةَ وَخَمْسَ عَشْرَةَ»
Abu Dharr reported God’s messenger as saying, “When you fast three days in the month, Abu Dharr, fast on the thirteenth, fourteenth and fifteenth.”
Mishkat al-Masabih 1966, Book 8, Fasting, Hadith 100. 1 min read