Musnad Abu Ya'la al-Mawsili 115:52

Hadbata narrated to us, Hammam narrated to us, Qatadah narrated to us, he said: I asked Anas, "Did the Messenger of Allah (may Allah bless him and grant him peace) dye his hair?" He said: "It did not reach that [extent]. His gray hair was only at his temples. However, Abu Bakr and Umar dyed [their hair] with henna and katam."