Mukhtasar Muslim

‘Sayyiduna Ibn Abbas (may Allah be pleased with them both) says that the People of the Book, that is, the Jews and Christians, used to let their hair hang down over their foreheads (that is, they did not part their hair), and the polytheists used to part their hair. And the Messenger of Allah (peace and blessings be upon him) liked to follow the way of the People of the Book in matters where he had not received any command (that is, compared to the polytheists, the People of the Book were better, so in matters where there was no command, the Prophet (peace and blessings be upon him) would adopt the practice of the People of the Book in that matter). So the Prophet (peace and blessings be upon him) also began to let his hair hang over his forehead, and after that, he began to part his hair.