The Prophet ﷺ used to like to conform with the People of the Book in matters concerning which he had received no instruction. The People of the Book would let their hair hang down over their foreheads, while the polytheists would part their hair. So the Prophet ﷺ let his forelock hang down, then later he parted his hair.
كان النبي ﷺ يحب موافقة أهل الكتاب فيما لم يؤمر فيه، وكان أهل الكتاب يسدلون أشعارهم، وكان المشركون يفرقون رؤوسهم، فسدل النبي ﷺ ناصيته، ثم فرق بعد.
Bukhari narrated it in the Book of Clothing (5917), and Muslim in the Book of Virtues (23361), both through the route of Ibrahim bin Saad, who said, Ibn Shihab narrated to us, from Ubaydullah bin Abdullah, from Ibn Abbas, and he mentioned it.His saying, "they let their hair hang down," means they let it fall loosely. And his saying, "they parted their heads," the parting of the hair is its division from the mafriq, which is the middle of the head, and the mafriq is the place where the hair divides, from the forehead to the crown of the head.