From Sahih Muslim (Hadith 6869):
Abu Bakr reported that he said to Allah's Messenger (ﷺ):Teach me a supplication which I should recite in my prayer. Thereupon he (the Holy Prophet) said: Recite:" O Allah, I have done great wrong to myself." According to Qutaiba (the words were: ) much (wrong) -there is none to forgive the sins but Thou only, say:" Grant me pardon from Thyself, have mercy upon me for Thou art much Forgiving and Compassionate
Reference: Muslim 6869