اللَّهُمَّ إِنِّي ظَلَمْتُ نَفْسِي ظُلْمًا كَثِيرًا، وَلَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ، فَاغْفِرْ لِي مَغْفِرَةً مِنْ عِنْدِكَ، وَارْحَمْنِي إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ
Allahumma inni zalamtu nafsi zulman kathiran, wa la yaghfirudh-dhunuba illa anta, faghfir li maghfiratan min 'indika, warhamni innaka antal-Ghafurur-Rahim
Allaahumma anigu waan isdhulmay nafta dulmiyid badan, midna ma cafiyo dambiyada adiga mooyee. Ee i cafee cafis ka yimid xaggaaga, ii naxariiso. Adigu waxaad tahay Cafiye, Naxariis leh.
The Prophet (PBUH) taught Abu Bakr to say this in salah
The Prophet (PBUH) taught this dua to Abu Bakr (RA) to say between tashahhud and salam in prayer.