Musnad Abu Ya'la al-Mawsili 115:81

Abu Bakr narrated to us, Zayd ibn al-Hubab narrated to us, from Ali ibn Mas'adah, Qatadah narrated to us, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Every son of Adam is a sinner, and the best of the sinners are those who repent."