Sahih al-Bukhari 5895Book 77, Hadith 112
حَدَّثَنَا سُلَيْمَانُ بْنُ حَرْبٍ، حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ، عَنْ ثَابِتٍ، قَالَ سُئِلَ أَنَسٌ عَنْ خِضَابِ النَّبِيِّ، صلى الله عليه وسلم فَقَالَ إِنَّهُ لَمْ يَبْلُغْ مَا يَخْضِبُ، لَوْ شِئْتُ أَنْ أَعُدَّ شَمَطَاتِهِ فِي لِحْيَتِهِ.
Anas was asked whether the Prophet (ﷺ) used a a hair dye or not. He replied, "The Prophet (ﷺ) did not have enough grey hair to dye, (such that) if I wanted to count the fading hairs in his beard (I could have)."