Musnad Abu Ya'la al-Mawsili 110:50

Hajjaj ibn Yusuf narrated to us, Ya'qub ibn Ibrahim narrated to us, he said: My father narrated to me, from Ibn Shihab, from 'Ubayd Allah ibn 'Abd Allah, from Ibn 'Abbas, who said: The People of the Scripture used to let their hair hang down, and the polytheists used to part their hair. The Messenger of Allah (may Allah bless him and grant him peace) liked to agree with the People of the Scripture in matters where he was not commanded otherwise. So, the Messenger of Allah (may Allah bless him and grant him peace) let his forelock hang down, then he parted it afterward.