We visited Umm Salama, and she brought out for us some of the Prophet's ﷺ hair, and it was dyed red with henna and katam.
حدثنا هاشم بن القاسم قال حدثنا أبو معاوية يعني شيبان
دخلنا على أم سلمة فأخرجت إلينا من شعر النبي ﷺ فإذا هو مخضوب أحمر بالحناء والكتم
Its isnad is sahih. Uthman bin Abdullah is Ibn Mawhab, and he is a well known trustworthy narrator whose hadith appears in the Sahihayn. The hadith was narrated by Bukhari 10/352 no. 5896, and Ibn Majah 2/1196 no. 3623, both in the chapter on clothing.