Musnad Ahmad

Once, Sayyiduna Abu Bakr (may Allah be pleased with him) submitted to the Prophet (peace and blessings be upon him), "O Messenger of Allah! Teach me a supplication that I may recite in my prayer." The Prophet (peace and blessings be upon him) taught him this supplication: «اللَّهُمَّ إِنِّي ظَلَمْتُ نَفْسِي ظُلْمًا كَثِيرًا ، وَلَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ ، فَاغْفِرْ لِي مَغْفِرَةً مِنْ عِنْدِكَ وَارْحَمْنِي ، إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ» "O Allah! I have greatly wronged myself, and none forgives sins except You, so forgive me with Your special grace and have mercy on me. Surely, You are the Most Forgiving, Most Merciful."