Sahih Bukhari
Ablutions (Wudu)
Volume: 001 / Book Number: 004 / Hadith Number: 152
Narrated 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.)"