Musnad Ahmad 32:1428

Yunus narrated to us, Hammad—meaning Ibn Zayd—narrated to us, from Thabit, that Anas was asked: "Did the Prophet, may Allah bless him and grant him peace, dye his hair?" He said: "The gray hairs of the Messenger of Allah, may Allah bless him and grant him peace, did not reach the point [that would require] dyeing. If I wished to count the white hairs that were in his beard, I could have done so." However, Abu Bakr used to dye with henna and katam, and Umar used to dye with henna.