Musnad Abu Ya'la al-Mawsili 117:90

Abu al-Rabi' narrated to us, Hammad narrated to us, from Thabit, who said: Anas was asked about the hair dye of the Messenger of Allah. He said: "If I wished to count the gray hairs on his head, I could have done so." And he said: "He did not dye his hair, whereas Abu Bakr dyed his hair with henna and katam, and Umar dyed his hair with henna."