Sahih al-Bukhari 3920
Sahih al-Bukhari · Book 63: Merits of the Helpers in Madinah (Ansaar) · The emigration of the Prophet (saws) to Al-Madina
Through another group of narrators, Anas bin Malik said:
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.
"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.