Musnad Ahmad 2919

It was narrated from Ibn ‘Abbas (رضي الله عنه) that The Prophet (ﷺ)used to let his hair hang down (over his forehead), and the mushrikoon used to part their hair. The People of the Book used to let their hair hang down over their foreheads and the Prophet (ﷺ) liked to do the same as the People of the Book in matters concerning which he had not received any revelation. Then (later on) the Messenger of Allah (ﷺ) parted his hair.