Sahih Bukhari Volume 007, Book 072, Hadith Number 787.
Narrated By Anas bin Malik : The Prophet was neither conspicuously tall, nor short; neither, very white, nor tawny. His hair was neither much curled, nor very straight. Allah sent him (as an Apostle) at the age of forty (and after that) he stayed for ten years in Mecca, and for ten more years in Medina. Allah took him unto Him at the age of sixty, and he scarcely had ten white hairs on his head and in his beard.