Hadithcore
Sahih al-Bukhari, 3920
sahih

Through another group of narrators, Anas bin Malik said:

"When the Prophet (ﷺ) arrived at Medina, the eldest amongst his companions was Abu Bakr. He dyed his hair with Hinna and Katam till it became of dark red color.

وَقَالَ دُحَيْمٌ حَدَّثَنَا الْوَلِيدُ، حَدَّثَنَا الأَوْزَاعِيُّ، حَدَّثَنِي أَبُو عُبَيْدٍ، عَنْ عُقْبَةَ بْنِ وَسَّاجٍ، حَدَّثَنِي أَنَسُ بْنُ مَالِكٍ ـ رضى الله عنه ـ قَالَ قَدِمَ النَّبِيُّ صلى الله عليه وسلم الْمَدِينَةَ، فَكَانَ أَسَنَّ أَصْحَابِهِ أَبُو بَكْرٍ، فَغَلَفَهَا بِالْحِنَّاءِ وَالْكَتَمِ حَتَّى قَنَأَ لَوْنُهَا‏.‏

References2 variants
In-Book Reference
Book 63, Hadith 145
USC-MSA web (English) reference
Vol. 5, Book 58, Hadith 257 (deprecated numbering scheme)