Musnad Ahmad 25:506

Ya'qub narrated to us, my father narrated to me, from al-Zuhri, from 'Ubayd Allah ibn 'Abd Allah, from Ibn 'Abbas, that he said: "The People of the Scripture used to let their hair hang down, and the polytheists used to part their hair." He said: "And the Messenger of Allah, may Allah bless him and grant him peace, liked to agree with the People of the Scripture in some matters regarding which he had not been commanded otherwise. So, the Messenger of Allah, may Allah bless him and grant him peace, let his forelock hang down." "Then he parted it afterwards."