Abu Bakr asked the Prophet Muhammad for a prayer to use when invoking Allah. The Prophet told him to say "O Allah! I have wronged my soul very much and none forgives sins but You, so please bestow Your Forgiveness upon me. No doubt, You are the Oft-Forgiving, Most Merciful."

2.  Narrated 'Abdullah bin 'Amr: Abu Bakr As-Siddiq said to the
Prophet "O Allah's Apostle! Teach me an invocation with which
Imay 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."
(Sahih Al-Bukhari, Vol. 9, Hadith 485)