Musnad Ahmad 2355

It was narrated from Ibn 'Abbas (رضي الله عنه) that he said: The people of the Book used to let their hair down and the mushrikeen used to part their hair. The Messenger of Allah (ﷺ) liked to do some of what the people of the Book did in some matters concerning which he had not received any commands, so the Messenger of Allah (ﷺ) let his forelock down, Then later on he parted his hair.