The Jews and Christians do not dye (their grey hair), so do the opposite of what they do
Authentic hadithعن أبي هريرة رضي الله عنه أن رسول الله صلى الله عليه وسلم قال: «إن اليهود والنصارى لا يصبغون، فخالفوهم».
Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah’s peace and blessings be upon him) said: “The Jews and Christians do not dye (their grey hair), so do the opposite of what they do.”
Explanation
Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) told the Companions that the Jews and Christians do not dye their hair or beards; rather, they leave grey hair as it is. So he ordered them to do the opposite of what the Jews and Christians do by dying their hair and beards.
Source · Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).