He traced it back to the Prophet ﷺ, who said, "The Jews and Christians do not dye their hair, so differ from them."
حدثنا مسدد حدثنا سفيان عن الزهري عن أبي سلمة وسليمان بن يسار
يبلغ به النبي ﷺ قال إن اليهود والنصارى لا يصبغون فخالفوهم
Bukhari reported it, Clothing, Chapter on Dyeing, no. 5899, and Muslim, Clothing, Chapter on Differing from the Jews in Dyeing, no. 2103, from the hadith of Sufyan bin Uyayna, with the same chain.