"Fasting three days of each month is fasting for a lifetime, and the shining days of Al-Bid, the thirteenth, fourteenth and fifteenth." (Using translation from Nasāʾī 2420)
عَنِ النَّبِيِّ ﷺ قَالَ «صِيَامُ ثَلَاثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ صِيَامُ الدَّهْرِ وَأَيَّامُ الْبِيضِ صَبِيحَةَ ثَلَاثَ عَشْرَةَ وَأَرْبَعَ عَشْرَةَ وَخَمْسَ عَشْرَةَ»