Hadith #815 (Sahih al-Bukhari)
Book: Call to Prayers (Adhaan)
Narrator(s): Abu Bakr As-Siddique
Grade: Sahih

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, “Allahumma inni zalumtu nafsi zulman kathiran, Wala yaghfiru dh-dhunuba illa anta, fa ghfir li maghfiratan min `indika, wa r-hamni, innaka anta l-ghafuru r-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).

Source: https://parallelquran.com/hadith/sahih-al-bukhari-815/