The hair of the Prophet ﷺ reached down to the middle of his ears.
حدثنا إسماعيل أخبرنا حميد الطويل
كان شعر النبي ﷺ إلى أنصاف أذنيه
No references are available.
Its isnad is sahih. It is also found in Muslim 4/1819, no. 2338, in the Book of Virtues, chapter on the description of the Prophet's ﷺ hair, and in Abu Dawud 4/81, no. 4186, in the Book of Combing the Hair, chapter on what has been said about hair, and in Nasai 8/133, no. 5061, in the Book of Adornment, chapter on keeping hair.