Musnad Ahmad 32:114

Ibn Abi ‘Adi narrated to us, from Humayd, who said: Anas was asked, "Did the Messenger of Allah, peace and blessings of Allah be upon him, dye his hair?" He said, "He did not have more than about seventeen or twenty white hairs in the front of his beard." And he said, "He was not disfigured by white hair." It was said to Anas, "Is it a disfigurement?" He said, "All of you dislike it, but Abu Bakr dyed his hair with henna and katam, and ‘Umar dyed his hair with henna."