Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1456
Qatada narrated it on the authority of Anas b. Malik that the Apostle of Allah (may peace be upon him) said: He who forgets tte prayer, or he slept (and it was omitted), its expiation is this only that he should observe it when he remembers it.