From Sahih Muslim (Hadith 1334):
Thauban reported:When the Messenger of Allah (ﷺ) finished his prayer. He begged forgiveness three times and said: O Allah! Thou art Peace, and peace comes from Thee; Blessed art Thou, O Possessor of Glory and Honour. Walid reported: I said to Auza'i: How is the seeking of forgiveness? He replied: You should say: I beg forgiveness from Allah, I beg forgiveness from Allah
Reference: Muslim 1334