From Sahih Bukhari (Hadith 7388):
Narrated `Abdullah bin `Amr:Abu Bakr As-Siddiq said to the Prophet (ﷺ) "O Allah's Messenger (ﷺ)! Teach me an invocation with which I may invoke Allah in my prayers." The Prophet (ﷺ) said, "Say: O Allah! I have wronged my soul very much (oppressed myself), and none forgives the sins but You; so please bestow Your Forgiveness upon me. No doubt, You are the Oft-Forgiving, Most Merciful
Reference: Bukhari 7388