Sahih al-Bukhari 3919
صحيح البخاري#3919SahihNarrated Anas:(the servant of the Prophet) When the Prophet (ﷺ) arrived (at Medina), there was not a single companion of the Prophet (ﷺ) who had grey and black hair except Abu Bakr, and he dyed his hair with Henna' and Katam (i.e. plants used for dying hair)
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 63:144