The hair of the Prophet ﷺ reached halfway down his ears.
أخبرنا علي بن حجر قال أنبأنا إسمعيل عن حميد
كان شعر النبي ﷺ إلى نصف أذنيه
No references are available.
Muslim narrated it in the Book of Virtues, Chapter on the Description of the Prophet's Hair, hadith no. 96/2337, from the hadith of Ismail bin Ulaya, with the same chain.