Jabir (May Allah be pleased with him) said: Abu Quhafah, father of Abu Bakr (May Allah be pleased with them) was presented to the Messenger of Allah (PBUH) on the day of the conquest of Makkah and his head and beard were snow white. The Messenger of Allah (PBUH) said, "Change it (i.e., dye it and avoid black colour)."[Muslim].
Abu Quhafah was the Kunyah (nick-name) of Abu Bakr's father. His real name was Usman bin `Amir. He had embraced Islam on the day Makkah was conquered by the Muslims. "Saghamah" is a herb which grows in mountains and is completely white. Since Abu Quhafah's hair were gray, the Prophet (PBUH) ordered him to dye them but forbade him from turning them black. Thus, we learn that except for inevitable circumstances, dying the hair of head and beard in black is prohibited.