Musnad Ahmad 32:692

Muhammad ibn Salamah al-Harrani narrated to us, from Hisham, from Muhammad ibn Sirin, who said: Anas ibn Malik was asked about the hair dye of the Messenger of Allah, peace and blessings of Allah be upon him. He said: "The Messenger of Allah, peace and blessings of Allah be upon him, did not have gray hair except for a little. However, Abu Bakr and Umar after him dyed [their hair] with henna and katam." He said: "Abu Bakr brought his father, Abu Quhafah, to the Messenger of Allah, peace and blessings of Allah be upon him, on the day of the conquest of Makkah, carrying him until he placed him before the Messenger of Allah, peace and blessings of Allah be upon him. The Messenger of Allah, peace and blessings of Allah be upon him, said to Abu Bakr: 'Had you left the old man in his house, we would have come to him, out of honor for Abu Bakr.'" So he accepted Islam, and his beard and head were as white as the *thaghamah* (a white plant). The Messenger of Allah, peace and blessings of Allah be upon him, said: "Change these two, and avoid black."