Musnad Ahmad 25:746

Ali ibn Ishaq narrated to us, he said: Abdullah and 'Attab informed us, they said: Abdullah narrated to us, he said: Yunus informed us, from al-Zuhri, he said: Ubaydullah narrated to me, from Ibn Abbas, that the Messenger of Allah (may Allah bless him and grant him peace) used to let his hair hang down, and the polytheists used to part their hair, and the People of the Scripture used to let their hair hang down. He liked to agree with the People of the Scripture in matters where he was not commanded with anything. Then, the Messenger of Allah (may Allah bless him and grant him peace) parted his hair.