Shaddad bin Aus (RAA) narrated that the Messenger of Allah (ﷺ) said:
"The best manner of asking for forgiveness is to say: "O Allah! You are my Lord. None has the right to be worshipped except You. You created me and I am your servant and I abide by your covenant and promise as best I can. I seek refuge in you from the evil, which I have committed. I acknowledge your favor upon me and I knowledge my sins, so forgive me, for verily none can forgive sin except you." Related by Al-Bukhari. (IT)
English reference (IT): Book 16, Hadith 1599 (IT)
Arabic reference (IT): Book 16, Hadith 1556 (IT)