Abu Bakr As-Siddiq (May Allah be pleased with him) reported:
I requested the Messenger of Allah (ﷺ) to teach me a supplication which I could recite in my Salat (prayer). Thereupon he said, "Recite: 'Allahumma inni zalamtu nafsi zulman kathiran, wa la yaghfirudh- dhunuba illa Anta, faghfir li maghfiratan min 'indika, warhamni, innaka Antal-Ghafur-ur-Rahim (O Allah! I have considerably wronged myself. There is none to forgive the sins but You. So grant me pardon and have mercy on me. You are the Most Forgiving, the Most Compassionate)."'[Al-Bukhari and Muslim].
App reference: Book 17, Hadith 11
Arabic/English book reference: Book 17, Hadith 1475