Musnad Abu Ya'la al-Mawsili 147:59

Abu Ubaydah ibn Fudayl ibn Iyad narrated to us, Abu Sa'id, the freed slave of Banu Hashim, narrated to us, Zarbi Abu Yahya narrated to us, he said: I heard Anas ibn Malik saying: The Messenger of Allah, may Allah bless him and grant him peace, said: "The most complete of the believers in faith are those of them with the best character."