Musnad Ahmad 25:371

Ishaq ibn Isa narrated to us, Ibrahim—meaning Ibn Sa’d—narrated to me, from al-Zuhri. Abdullah ibn Ahmad said: My father said: And Ya’qub narrated to me, from my father, from Ibn Shihab, from Ubaydullah ibn Abdullah, from Ibn Abbas, who said: "The polytheists used to part their hair, and the People of the Scripture used to let their hair hang down—Ya’qub said: their hair—and the Messenger of Allah, may Allah bless him and grant him peace, liked and was pleased with conforming to the People of the Scripture—Ya’qub said: in some of what he was not commanded [to do]. Ishaq said: in what he was not commanded [to do]—so he let his forelock hang down, then he parted it afterwards."