Abu Bakr as-Siddiq said that he asked God’s Messenger to teach him a supplication to use in his prayer, and he told him to say, “O God, I have greatly wronged myself, and You alone canst forgive sins, so grant me forgiveness from You and show mercy to me. You are the forgiving and the merciful One.” (Bukhari and Muslim.)

وَعَنْ أَبِي بَكْرٍ الصِّدِّيقِ رَضِيَ اللَّهُ عَنْهُ أَنَّهُ قَالَ: قُلْتُ يَا رَسُولَ اللَّهِ عَلِّمْنِي دُعَاءً أَدْعُو بِهِ فِي صَلَاتِي قَالَ: «قُلْ اللَّهُمَّ إِنِّي ظَلَمْتُ نَفْسِي ظُلْمًا كَثِيرًا وَلَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ فَاغْفِرْ لِي مَغْفِرَةً مِنْ عنْدك وارحمني إِنَّك أَنْت الغفور الرَّحِيم»