Musnad Ahmad 32:1135

Yazid ibn Harun narrated to us, Humaid informed us, he said: Anas was asked: "Did the Messenger of Allah, may Allah bless him and grant him peace, dye his hair?" He said: "Gray hair did not disfigure him." It was said: "Was it disfigured?" He said: "All of you dislike it. They were only a few hairs at the front of his beard." And Humaid pointed to the front of his own beard.