Abu Dawud Book 28, Hadith Number 4171.
Chapter : Extent to which hair should hang.
Narated By Al Bara’ : I did not see any man with locks hanging down to shoulders in a red robe more beautiful than tha Apostle of Allah (PBUH). Muhammad b. Sulaiman added: He had hair which touched his shoulders.
Abu Dawud said: Isra’il also transmitted it in a similar way from Abu Ishaq saying: (his hair) touched his shoulders.” Shu’bah said: (His hair) reached the lobes of his ears.