Musnad Abu Ya'la al-Mawsili 115:93

Abu Hamza Huraym narrated to us, Mu'tamir narrated to us, from his father, Qatada narrated to us, from Anas bin Malik, from the Messenger of Allah (may Allah bless him and grant him peace), that he used to say: "Verily, I seek forgiveness seventy times in a day."