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. (IT)
English reference (IT): Book 5, Hadith 704 (IT)
Arabic reference (IT): Book 5, Hadith 684 (IT)