Praise be to Allah,
Mu’tamir narrated to us from Humayd [at-I aweel] from Anas ibn Maalik who said,
“There were not even twenty grey hairs on the head and beard of the Messenger of Allah. Abu Bakr dyed [his beard] with henna and katam, while Umar dyed his with henna .”
(Musnad Ahmad} Vol. 3, p. 100. The isnaad of this Hadeeth is saheeh according to the conditions set by al-Bukhaaree and Muslim.)
Henna (Lawsonia inermis) is a flowering plant used since antiquity to dye skin, hair, fingernails, leather, and wool. It is common today as a reddish-brown dye used especially on the hair and skin. The Latin name of katam is Buxus dioica, which is a plant that grows only in the high mountains. It strengthens the hair and darkens the color of henna and makes it last longer.
Thulaathiyyaat Musnad Ahmad ibn Hanbal (may Allah have mercy on him)
Reports of Imaam Ahmad that have only three narrators between him and the Prophet, peace, and blessings be upon him.
Shaykh Muhammad ibn Ahmad ibn Saalim as–Saffaareenee al–Hanbalee ~(1114 –1188 AH)