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

O Allāh, I have indeed oppressed my soul excessively and none can forgive sin except You, so forgive me a forgiveness from Yourself and have mercy upon me. Surely, You are The Most-Forgiving, The Most-Merciful.

Reference: Al-Bukhārī (#7387, 8/168) and Muslim (#2705, 4/2078).