Musnad Ahmad 1018:89

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Husayn, from Hilal ibn Yasaf, from Zadhan, from a man among the Companions of the Prophet, may Allah bless him and grant him peace, from the Ansar. Shu‘bah said, or he said: A man from the Ansar, that he heard the Prophet, may Allah bless him and grant him peace, in prayer, and he was saying: "My Lord, forgive me." Shu‘bah said: Or he said: "O Allah, forgive me and accept my repentance, for You are the Accepting of Repentance, the Forgiving," one hundred times.