The hair of the Messenger of Allah ﷺ did not pass beyond his ears.
حدثنا عبد الصمد حدثنا حماد عن ثابت
كان شعر رسول الله ﷺ لا يجاوز أذنيه
No references are available.
Its isnad is sahih, and it is found in Muslim 4/1819, no. 2338, in the Book of Virtues, chapter on the description of the Prophet's hair ﷺ, and in Abu Dawud 4/81, no. 4186, in the Book of Grooming, chapter on what has been said about hair.