Sahih Bukhari Volume 5, Book 58, Hadith Number 257.

Narated By Anas : (The servant of the Prophet) When the Prophet arrived (at Medina), there was not a single companion of the Prophet who had grey and black hair except Abu Bakr, and he dyed his hair with Henna’ and Katam (i.e. plants used for dying hair). Through another group of narrators, Anas bin Malik said,. “When the Prophet arrived at Medina, the eldest amongst his companions was Abu Bakr. He dyed his hair with Hinna and Katam till it became of dark red color.