Musnad Ahmad 640:1

Mu'tamir narrated to us, he said: I heard Ayyub, and Muhammad ibn Abd al-Rahman al-Tafawi narrated to us, he said: Ayyub al-Ma'ni narrated to us, from Humayd ibn Hilal, from Abu Burdah, from a man from the Emigrants (Muhajirun), who said: I heard the Prophet, peace and blessings of Allah be upon him, saying: "O people, repent to Allah and seek His forgiveness, for I repent to Allah and seek His forgiveness every day one hundred times, or more than one hundred times."