بُلُوغُ الْمَرَامِ
Bulugh al-Maram 319
The Book of Prayer
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]
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].
Sunnah.com reference: Book 2, Hadith 212