Sahih Bukhari
Virtues and Merits of the Prophet (pbuh) and his Companions
Volume: 004 / Book Number: 056 / Hadith Number: 668
Narrated 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)."