Sahih Muslim | Book 30 | Hadith Number 5768

Narrated by Ibn Abbas
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 (may peace be upon him) 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 (may peace be upon him) let fall his hair upon his forehead, and then he began to part it after this.