Musnad Ahmad 490:7

Abdullah narrated to us, Muhammad ibn Abdullah al-Mukharrimi narrated to us, Abu Sufyan al-Himyari Sa'id ibn Yahya narrated to us, he said: al-Dahhak ibn Hamzah narrated to us, from Ghaylan ibn Jami', from Iyad ibn Laqit, from Abu Rimthah, who said: "The Prophet, may Allah bless him and grant him peace, used to dye [his hair/beard] with henna and katam. And his hair would reach his shoulders or his shoulder joints."