اَللَّهُمَّ إِنِّي ظَلَمْتُ نَفْسِي ظُلْمًا كَثِيْرًا وَلَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ فَاغْفِرْ لِي مَغْفِرَةً مِّنْ عِنْدِكَ وَارْحَمْنِيْ إِنَّكَ أَنْتَ الْغَفُوْرُ الرَّحِيْمُ

Allaahumma inni zalamtu nafsi zulman kathiran wa laa yaghfirudh-dhunuba illaa Anta, faghfir li maghfiratan min 'indika warhamni, innaka Antal-Ghafurur-Rahim

O Allah! I have done great injustice to myself and none except You forgives sins, so bestow on me a forgiveness from You, and Have Mercy on me, You are the Forgiver, the Merciful. Narrated by Abu Bakr As-Siddiq (RA): I asked Allah's Messenger (ﷺ) to teach me an invocation so that I may invoke Allah with it in my prayer. He told me to say: (The dua is mentioned above)

Reference: Bukhari: 834