Narrated 'Abdullah bin 'Amr: Abu Bakr As-Siddiq said to the Prophet(saws), "Teach me an invocation with which I may invoke (Allah) in my prayer." The Prophet said(saws), "Say: Allahumma inni zalamtu nafsi zulman kathiran wala yaghfirudh-dhunuba illa anta, Faghfirli maghfiratan min indika war-hamni, innaka antalGhafur-Rahim."
Translation :O Allah, indeed I have wronged myself greatly, and there is none who forgives sins besides You. So grant me forgiveness from You and have mercy on me, for you are the All-Forgiving, Most Merciful.
Sahih Bukhari