Musnad Ahmad 32:1718

'Affan narrated to us, Hammad narrated to us, from Thabit, who said: It was said to Anas: "Did the Messenger of Allah, peace and blessings of Allah be upon him, have gray hair?" He said: "Allah did not disfigure him with gray hair; there were not in his head and beard except seventeen or eighteen [hairs]."