سنن ابن ماجه

It was narrated from Abu Hurairah that the Prophet (ﷺ) used to fast on Mondays and Thursdays. It was said: “O Messenger of Allah, why do you fast on Mondays and Thursdays?” He said: “On Mondays and Thursdays Allah forgives every Muslim except two who have forsaken one another. He says: ‘Leave these two until they reconcile.’”