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.