Musnad Ahmad 25:1083

Uthman ibn Umar narrated to us, Yunus narrated to us, from al-Zuhri, from Ubayd Allah ibn Abd Allah, from Ibn Abbas: That the Prophet, peace and blessings of Allah be upon him, used to let his hair hang down, and the polytheists used to part their heads, and the People of the Scripture used to let their heads hang down. The Prophet, peace and blessings of Allah be upon him, liked to agree with the People of the Scripture in matters where nothing had been revealed to him. So, the Messenger of Allah, peace and blessings of Allah be upon him, parted his head.