Musnad Ahmad 1051:1

Isma'il narrated to us, Yunus narrated to us, from Humayd ibn Hilal, from Abu Burdah, who said: I sat with an old man from the companions of the Prophet, may God bless him and grant him peace, in the mosque of Kufa, and he narrated to me. He said: I heard the Messenger of God, may God bless him and grant him peace—or he said: The Messenger of God, may God bless him and grant him peace, said: "O people, repent to God and seek His forgiveness, for I repent to God and seek His forgiveness one hundred times every day." I said: "O God, I seek Your forgiveness, [O God, I repent to You]," two [times]. He said: "It is what I am telling you."