Reference : Hisn al-Muslim 57

اللّهُـمَّ إِنِّـي ظَلَـمْتُ نَفْسـي ظُلْمـاً كَثـيراً 
وَلا يَغْـفِرُ الذُّنـوبَ إِلاّ أَنْت، 
فَاغْـفِر لي مَغْـفِرَةً مِنْ عِنْـدِك وَارْحَمْـني، 
إِنَّكَ أَنْتَ الغَـفورُ الرَّحـيم
Allāhumma ‘innī ẓalamtu nafsī ẓulman kathīran, 
wa lā yaghfiru-dhdhunūba illā 'anta, 
faghfir lī maghfiratam’min `indika warḥamnī 
innaka 'anta ‘l-Ghafūr ur-Rahīm.


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.

Reference:
Al-Bukhari 8/168, Muslim 4/2078.

Chapter: After the last tashahhud and before salam