Musnad Ahmad 32:885

Sahl ibn Yusuf narrated to us, from Humayd, who said: Anas ibn Malik was asked, "Did the Messenger of Allah, peace and blessings of Allah be upon him, dye his hair?" He said, "No, the gray hair did not disfigure him." It was said, "O Abu Hamzah, is it a disfigurement?" He said, "It is said: all of you dislike it." And Abu Bakr dyed his hair with henna and katam, and Umar dyed his hair with henna.