Musannaf Ibn Abi Shayba 33:848

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