The Prophet (ﷺ) said, "Jews and Christians do not dye their hair so you should do the opposite of what they do.
Sahih al-Bukhari Book 77, Hadith 116
← Previous Hadith Ibn Mauhab also said that Um Salama had shown him the red hair of the Prophet.

Sahih al-Bukhari Book 77, Hadith 115 Next Hadith → The Prophet (ﷺ) was neither conspicuously tall, nor short; neither, very white, nor tawny. His hair was neither much curled, nor very straight. Allah sent him (as an Apostle) at the age of forty (and after that) he stayed for ten years in Mecca, and for ten more years in Medina. Allah took him unto Him at the age of sixty, and he scarcely had ten white hairs on his head and in his beard.

Sahih al-Bukhari Book 77, Hadith 117