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.
— Sahih Bukhari, Volume 1, Book 9: Virtues of the Prayer Hall (Sutra of the Musalla), Hadith #479