Musnad Ahmad 32:1686

'Affan narrated to us, Hammam narrated to us, Qatadah narrated to us, he said: I said to Anas bin Malik: "Did the Messenger of Allah, peace and blessings of Allah be upon him, dye his hair?" He said: "It did not reach that [extent]; it was only a little in his temples. But Abu Bakr dyed [his hair] with henna and katam."