Sahih Al Bukhari – Book of Virtues And Merits Of The Prophet (pbuh) And His Companions
Volumn 004, Book 056, Hadith Number 668.
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Narated By Abu Huraira RA: Allah's Apostle PBUH 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)."
