From Sahih Muslim (Hadith 6062):
Ibn Abbas reported that the People of the Book used to let their hair fall (on their foreheads) and the polytheists used to part them on their heads, and Allah's Messenger (ﷺ) liked to conform his behaviour to the People of the Book in matters in which he received no command (from God) ; so Allah's Messenger (ﷺ) let fall his hair upon his forehead, and then he began to part it after this
Reference: Muslim 6062