Abu Dharr (RAA) narrated that The Messenger of Allah (ﷺ) 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-Nasa’i and At-Tirmidhi and rendered authentic by Ibn Hibban.

وَعَنْ أَبِي ذَرٍّ ‏- رضى الله عنه ‏- قَالَ: { أَمَرَنَا رَسُولُ اَللَّهِ ‏- صلى الله عليه وسلم ‏-أَنْ نَصُومَ مِنْ اَلشَّهْرِ ثَلَاثَةَ أَيَّامٍ: ثَلَاثَ عَشْرَةَ, وَأَرْبَعَ عَشْرَةَ وَخَمْسَ عَشْرَةَ } رَوَاهُ النَّسَائِيُّ, وَاَلتِّرْمِذِيُّ, وَصَحَّحَهُ اِبْنُ حِبَّانَ [1]‏ .‏