It was narrated that Anas said:
"The Messenger of Allah (ﷺ) was asked about a man who slept and missed the prayer, or forgot it. He said: 'The expiation for that is to pray it when he remembers it.'" (ID)
English reference (ID): Vol. 1, Book 6, Hadith 615 (ID)
Arabic reference (ID): Book 6, Hadith 621 (ID)