Musannaf Ibn Abi Shayba 21:146

Ibn Fudayl narrated to us, from Husayn, from Hilal ibn Yasaf, from Zadhan, who said: A man from the Ansar narrated to me, saying: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying at the end of the prayer: "O Allah, forgive me and accept my repentance, for You are the Oft-Returning, the Most Merciful," one hundred times.