Musnad Ahmad 490:10

Abdullah narrated to us, Muhammad ibn Hassan al-Azraq narrated to us, Abu Sufyan al-Himyari narrated to us, al-Dahhak ibn Hamzah narrated to us, from Ghaylan ibn Jami’, from Iyad ibn Laqit, from Abu Rimthah, who said: "The Prophet, peace and blessings of Allah be upon him, used to dye [his hair/beard] with henna and katam. His hair would reach his shoulders or his shoulder joints." (Abu Sufyan was in doubt).