Musnad Ahmad 27:1115

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Yunus ibn Khabbab, Abu al-Fadl or Ibn al-Fadl narrated to us, from Ibn 'Umar, that he was sitting with the Messenger of Allah, may Allah bless him and grant him peace, and he said: "O Allah, forgive me and accept my repentance, for You are the Accepting of Repentance, the Forgiving." He repeated this until the counter counted one hundred times on his hand.