Musannaf Ibn Abi Shayba 35:47

Hushaym narrated to us, from Ayyub, from Abu al-Ala, Qatada narrated to us, from Anas, who said: The Prophet (peace and blessings of Allah be upon him) said: "Whoever forgets a prayer or sleeps through it, then its expiation is that he prays it when he remembers it."