Qatada narrated it on the authority of Anas bin Malik (RA) that the Apostle of Allah (ﷺ) said: He who forgets the prayer, or he slept (and it was omitted), its expiation is this only that he should observe it when he remembers it.
Hadith ReferenceSahih Muslim / The Book of Mosques and Places of Prayer / Hadith: 684