Duaa #2 In tashahhud & before end of the prayer:

اللَّهُمَّ إِنِّي ظَلَمْتُ نَفْسِي ظُلْماً كَثِيراً، وَلَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ، فَاغْفِرْ لِي مَغْفِرَةً مِنْ عِنْدِكَ وَارْحَمْنِي إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ

Allaahumma 'innee dhalamtu nafsee dhulman katheeran, wa laa yaghfiruth-thunooba 'illaa 'Anta, faghfir lee maghfiratan min 'indika warhamnee 'innaka 'Antal-Ghafoorur-Raheem

O Allah, I have greatly wronged myself and no one forgives sins but You. So grant me forgiveness and have mercy on me. Surely, you are Forgiving, Merciful.

Al-Bukhari 8/168, Muslim 4/2078