Narrated Abu Bakr as-Siddiq (RA):

He said to Allah's Messenger (ﷺ), "Teach me a supplication to use in my prayer." He (ﷺ) said: "Say: O Allah, I have greatly wronged myself, and no one forgives sins except You, so grant me forgiveness from You and have mercy on me, You are the Forgiving and the Merciful One." [Agreed upon].

وَعَنْ أَبِي بَكْرٍ اَلصِّدِّيقِ ‏- رضى الله عنه ‏- { أَنَّهُ قَالَ لِرَسُولِ اَللَّهِ ‏- صلى الله عليه وسلم ‏-عَلِّمْنِي دُعَاءً أَدْعُو بِهِ فِي صَلَاتِي .‏ قَالَ قُلْ : " اَللَّهُمَّ إِنِّي ظَلَمْتُ نَفْسِي ظُلْمًا كَثِيرًا , وَلَا يَغْفِرُ اَلذُّنُوبَ إِلَّا أَنْتَ , فَاغْفِرْ لِي مَغْفِرَةً مِنْ عِنْدِكَ , وَارْحَمْنِي , إِنَّكَ أَنْتَ اَلْغَفُورُ اَلرَّحِيمُ } مُتَّفَقٌ عَلَيْهِ .‏ [1]‏