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Hammad said: 'Abdullah ibn Muhammad ibn 'Uqail also informed us: "I saw the hair of Allah’s Messenger (Allah bless him and give him peace) dyed in the presence of Anas ibn Malik."
Source: sunnah.com
Other translations
Arabic
قَالَ حَمَّادٌ : وَأَخْبَرَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدِ بْنِ عَقِيلٍ قَالَ : رَأَيْتُ شَعْرَ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ عِنْدَ أَنَسِ بْنِ مَالِكٍ مَخْضُوبًا
sunnah.com