It wasnarrated from Abu Hurairah that the Prophet (ﷺ) used to fastonMondays and Thursdays. It was said:
"O Messenger of Allah, why doyou fast on Mondays and Thursdays?" He said: "On Mondays andThursdaysAllah forgives every Muslim except two who have forsakenone another.He says: 'Leave these two until they reconcile.'" (JA)
English reference (JA): Vol. 1, Book 7, Hadith 1740 (JA)
Arabic reference (JA): Book 7, Hadith 1812 (JA)