Musnad Ahmad 32:1200

Rawh narrated to us, Hisham narrated to us, from Muhammad, who said: I asked Anas bin Malik: "Did the Messenger of Allah, peace and blessings of Allah be upon him, dye his hair?" He said: "He did not have—meaning of gray hair—except a little. And Abu Bakr and Umar dyed [their hair], I believe with henna and katam."