Musnad Ahmad 27:279

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