Sahih Al Bukhari – Book of Invocations

Volumn 008, Book 075, Hadith Number 338.
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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.”