It was narrated from Ibn 'Abbas (رضي الله عنه) that

The Messenger of Allah (ﷺ) used to let his hair hang down. The mushrikeen used to part their hair and the people of the Book used to let their hair hang down, and he liked to do the same as the people of the Book in matters concerning which he had received no command. Then the Messenger of Allah (ﷺ) parted his hair.