It was narrated from 'Abdullah bin 'Amr, from Abu Bakr As-Siddiq, may Allah (SWT) be pleased with them both, that he said to the Messenger of Allah (ﷺ):
"Teach me a supplication that I may recite in my prayer." He said: "Say: 'Alahumma inni zalamtu afsi zulman kathiran wa la yaghfirudhunub illa anta faghfirli maghfiratan min 'indika warhamni innaka antalGhafurur-Rahim (O Allah, verily I have wronged myself much and there is None who forgives sins except You. Grant me forgiveness from You and have mercy on me for You are the Oft-Forgiving, Most Merciful.)'"