Musnad Ahmad 32:1813

Hassan narrated to us, Muhammad ibn Rashid narrated to us, from Makhul, from Musa ibn Anas, from his father, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, did not reach the amount of gray hair that would require him to dye it. However, Abu Bakr dyed his head and his beard until his hair became dark red with henna and katam."