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.