Sunan an-Nasa'i, Volume 2, Book 13, Hadith 1303 (DarusSalam) ⓘ

It was narrated from 'Abdullah bin 'Amr, from Abu Bakr As-Siddiq, may Allah (SWT) be pleased with them both, that he said to the Messenger of Allah: "Teach me a supplication that I may recite in my prayer." He said: "Say: 'Alahumma inni zalamtu afsi zulman kathiran wa la yaghfirudhunub illa anta faghfirli maghfiratan min 'indika warhamni innaka antalGhafurur-Rahim (O Allah, verily I have wronged myself much and there is None who forgives sins except You. Grant me forgiveness from You and have mercy on me for You are the Oft-Forgiving, Most Merciful.)'"

أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ يَزِيدَ بْنِ أَبِي حَبِيبٍ، عَنْ أَبِي الْخَيْرِ، عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو، عَنْ أَبِي بَكْرٍ الصِّدِّيقِ، - رضى الله عنهما أَنَّهُ قَالَ لِرَسُولِ اللَّهِ صلى الله عليه وسلم عَلِّمْنِي دُعَاءً أَدْعُو بِهِ فِي صَلاَتِي ‏.‏ قَالَ ‏ "‏ قُلِ اللَّهُمَّ إِنِّي ظَلَمْتُ نَفْسِي ظُلْمًا كَثِيرًا وَلاَ يَغْفِرُ الذُّنُوبَ إِلاَّ أَنْتَ فَاغْفِرْ لِي مَغْفِرَةً مِنْ عِنْدِكَ وَارْحَمْنِي إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ ‏" ‏ ‏.‏