Sahih Bukhari Volume 8, Book 75, Hadith Number 338.

Narated By ‘Abdullah bin ‘Amr : Abu Bakr As-Siddiq said to the Prophet, “Teach me an invocation with which I may invoke (Allah) in my prayer.” The Prophet said, “Say: Allahumma inni zalamtu nafsi zulman kathiran wala yaghfirudh-dhunuba illa anta, Faghfirli maghfiratan min indika war-hamni, innaka antalGhafur-Rahim.”