Narrated by Abu Bakr As-Siddiq (RA): I asked Allah's Messenger (ﷺ) to teach me a dua so that I may invoke Allah with it in my prayer. He told me to say:
اَللَّهُمَّ إِنِّي ظَلَمْتُ نَفْسِيْ ظُلْمًا كَثِيْرًا وَلَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ فَاغْفِرْ لِي مَغْفِرَةً مِنْ عِنْدِكَ وَارْحَمْنِيْ إِنَّكَ أَنْتَ الْغَفُوْرُ الرَّحِيْمُ
اَللّٰهُمَّ اِنِّىْ ظَلَمْتُ نَفْسِىْ ظُلْمًا كَثِيْرًا وَّلَا يَغْفِرُ الذُّنُوْبَ اِلَّا اَنْتَ فَاغْفِرْ لِىْ مَغْفِرَةً مِّنْ عِنْدِكَ وَارْحَمْنِىْ اِنَّكَ اَنْتَ الْغَفُوْرُ الرَّحِيْمُ
Allaahumma 'innee dhalamtu nafsee dhulman katheeran, wa laa yaghfirudh-dhunooba '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.