وَعَنْ أَبِي ذَرٍّ رضى الله عنه
قَالَ: «أَمَرَنَا رَسُولُ اَللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: أَنْ نَصُومَ مِنْ اَلشَّهْرِ ثَلَاثَةَ أَيَّامٍ: ثَلَاثَ عَشْرَةَ، وَأَرْبَعَ عَشْرَةَ وَخَمْسَ عَشْرَةَ»
The Messenger of Allah (sallallahu alayhi wa sallam) commanded us to fast for three days of every month; that is on the days of the full moon; the 13th, 14th, and 15th (of the lunar month). Related by An-Nasai and At-Tirmidhi and rendered authentic by Ibn Hibban.
Bulugh al-Maram 792, Book 5, Fasting, Hadith 35. 1 min read