Musnad Abu Ya'la al-Mawsili 117:220

Al-Jarrah ibn Makhlad narrated to us, Ismail ibn Abd al-Hamid ibn Abd al-Rahman al-Ijli narrated to us, Ali ibn Abi Sarah narrated to us, from Thabit, from Anas: That a man entered upon the Prophet, peace and blessings of Allah be upon him, with a white head and beard. He said, "Are you not a Muslim?" He said, "Yes." He said, "Then dye [your hair]."