Virtues of the Prayer Hall (Sutra of the Musalla)
Bukhari :: Book 1 :: Volume 9 :: Hadith 479
Narrated Anas Ibn Malik:
Whenever the Prophet went for answering the call of nature, I and another boy used to go after him with a staff, a stick or an 'Anza and a tumbler of water and when he finished from answering the call of nature we would hand that tumbler of water to him.
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