Musannaf Ibn Abi Shayba 21:325

Al-Fadl ibn Dukayn narrated to us, Mughira ibn Abi al-Hurr narrated to us, from Sa'id ibn Abi Burda, from his father, from his grandfather, who said: The Messenger of Allah (may Allah bless him and grant him peace) came while we were sitting, and he said: "There is no morning that I have reached except that I have sought Allah's forgiveness in it one hundred times."