Musannaf Ibn Abi Shayba 16:844

Abu Bakr narrated to us, he said: Yahya ibn Adam narrated to us, from Ibrahim ibn Sa’d al-Zuhri, 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 follow the People of the Scripture in matters where he had not been commanded otherwise. He said: So the Messenger of Allah (may Allah bless him and grant him peace) let his forelock hang down, then he parted it afterwards.