Narrated Abu Bakr As-Siddiq:
I asked Allah's Apostle to teach me an invocation so that I may invoke Allah with it in my prayer. He told me to say, "Allahumma inni zalumtu nafsi zulman kathiran, Wala yaghfirudhdhunuba illa anta faghfirli maghfiratan min 'Indika, war-hamni innaka antal-ghafururrahim (O Allah! I have done great injustice to myself and none except You forgives sins, so please forgive me and be Merciful to me as You are the Forgiver, the Merciful)."