sahih bukhari volume 1, Book 4, Number 153:
Narrated Anas:
Whenever Allah's Apostle 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.
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Volume 1, Bo ok 4, Number 154:
Narrated Anas bin Malik:
Whenever Allah's Apostle went to answer the call of nature, I along with another boy used to carry a tumbler full of water (for cleaning the private parts) and an 'Anza (spear-headed stuck).