The Messenger of Allah ﷺ used to let his forelock hang down for as long as Allah willed, then later he parted his hair.
حدثنا حماد بن خالد حدثنا مالك حدثنا زياد بن سعد عن الزهري
سدل رسول الله ﷺ ناصيته ما شاء الله أن يسدلها ثم فرق بعد
Its isnad is sahih. Hammad bin Khalid and Ziyad bin Saad are both reliable and firmly established narrators. The hadith was narrated by Bukhari 4/230 in the chapter on Virtues, description of the Prophet ﷺ, Muslim 4/1817 no. 2336 in the chapter on Virtues, on the Prophet ﷺ letting his hair fall loose, and Abu Dawud 4/82 no. 4188.