Sahih Bukhari
Sahih Bukhari Volume 001, Book 004, Hadith Number 152.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 04. Ablutions (Wudu')
Narated By Anas bin Malik : Whenever Allah's Apostle 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.)"