Musnad Abu Ya'la al-Mawsili 110:228

Zuhayr narrated to us, Uthman ibn Umar narrated to us, Yunus informed us, from al-Zuhri, from Ubayd Allah ibn Abd Allah, from Ibn Abbas, that the Messenger of Allah (may Allah bless him and grant him peace) used to let his hair hang down (sadl). The polytheists used to part their hair, and the People of the Scripture used to let their hair hang down. The Messenger of Allah (may Allah bless him and grant him peace) liked to agree with the People of the Scripture in matters where nothing had been revealed to him. Then, the Messenger of Allah (may Allah bless him and grant him peace) parted his hair.