Sahih Bukhari Volume 004, Book 056, Hadith Number 668.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 56. Virtues and Merits of the Prophets (PBUH) and his Companions
Narated By Abu Huraira : Allah's Apostle said, "The Jews and the Christians do not dye (their grey hair), so you shall do the opposite of what they do (i.e. dye your grey hair and beards)."