Musnad Ahmad 907:6

Sulayman ibn Dawud narrated to us, Shu'bah informed us, from Simak, who said: I heard Jabir ibn Samurah, and he was asked about the gray hair of the Prophet, may Allah bless him and grant him peace. He said: "There were a few hairs on his head; when he oiled his head, they would not be apparent, and when he did not oil it, they would be apparent."