Yahya bin Khlf al-Bahly يحيى بن خلف الباهلي أبو سلمة البصري · Combing the Hair (Kitab Al-Tarajjul)

The people of the Book used to let their hair hand down, and the polytheists used to part their hair. The Messenger of Allah (ﷺ) like to confirm with the People of the Book in the matters about which he had received no command. Hence he Messenger of Allah (ﷺ) let his forelock hang down but afterwards he parted it.