Musannaf Ibn Abi Shayba 21:323

Abdullah ibn Numayr narrated to us, Malik ibn Mighwal narrated to us, from Muhammad ibn Suqah, from Nafi’, from Ibn Umar, who said: We used to count for the Messenger of Allah, may Allah bless him and grant him peace, in a single sitting, saying one hundred times: "My Lord, forgive me and accept my repentance, for You are the Accepting of Repentance, the Forgiving."