(15) باب الاِسْتِنْجَاءِ بِالْمَاءِ
Narrated 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.)"
Reference: Sahih al-Bukhari 150
In-book reference: Book 4, Hadith 16
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 4, Hadith 152