Sahih Al Bukhari – Book of Oneness, Uniqueness Of Allah (Tawheed)

Volumn 009, Book 093, Hadith Number 485. 
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Narated By 'Abdullah bin 'Amr RA: Abu Bakr As-Siddiq RA said to the Prophet PBUH"O Allah's Apostle! Teach me an invocation with which I may invoke Allah in my prayers." The Prophet said, "Say: O Allah! I have wronged my soul very much (oppressed myself), and none forgives the sins but You; so please bestow Your Forgiveness upon me. No doubt, You are the Oft-Forgiving, Most Merciful."