Sahih al-Bukhari 150
صحيح البخاري#150SahihNarrated Anas bin Malik:Whenever Allah's Messenger (ﷺ) went to answer the call of nature, I along with another boy used to accompany him with a tumbler full of water. (Hisham commented, "So that he might wash his private parts with it)
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: 4:16