Musannaf Ibn Abi Shayba 21:113

Ubaydah ibn Humayd narrated to us, from Ayyub, from Abu Fakhitah, from Mujahid, who said: Abu Sa’id al-Khudri said: "No man places his forehead on the ground in prostration to Allah, then says, 'O Lord, forgive me; O Lord, forgive me; O Lord, forgive me'—three times—except that he raises his head having been forgiven."