He traced it back to the Prophet ﷺ, who said, "The Jews and Christians do not dye their hair, so differ from them."
حدثنا أبو بكر حدثنا سفيان بن عيينة عن الزهري سمع أبا سلمة وسليمان بن يسار يخبران
يبلغ به النبي ﷺ قال إن اليهود والنصارى لا يصبغون فخالفوهم
Bukhari transmitted it in "Clothing," chapter "Dyeing the Hair," Hadith 5899, from the hadith of Sufyan, through this route, and Muslim transmitted it in "Clothing," chapter "On Differing from the Jews in Dyeing," Hadith 2103, from Abu Bakr bin Abu Shayba, through this route.