Anas Radiyallahu 'Anhu reports:
"Rasulullah Sallallahu 'Alayhi Wasallam was not of a tall height nor was he short. (According to colour) he was not very white, nor very wheat coloured (dark). His mubaarak hair was not very curly nor very straight (but was slightly curled). He was blessed with prophethood at the age of forty, after that he lived for ten years in Makkah Mukarramah, and ten years in Madinah Munawwarah. At the age of sixty years Rasulullah Sallallahu 'Alayhi Wasallam passed away. At that time he did not have more than twenty white hair in his mubaarak head and beard".
English reference: Book 52, Hadith 367
Arabic reference: Book 53, Hadith 383