Sahih al-Bukhari 151
صحيح البخاري#151SahihNarrated Anas:Whenever Allah's Messenger (ﷺ) went to answer the call of nature, I along with another boy from us used to go behind him with a tumbler full of water
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: 4:17