Musnad Abu Ya'la al-Mawsili 7:29

Ya'qub ibn Ibrahim narrated to us, Abu 'Asim narrated to us, from Ja'far ibn Muhammad, from his father, who said: 'Umar ibn al-Khattab said, "What shall I do regarding the Magians?" 'Abd al-Rahman ibn 'Awf stood up and said, "I heard the Messenger of Allah, peace and blessings of Allah be upon him, when he was asked about them, say: 'Their tradition is the tradition of the People of the Book.'"