Sahih Muslim | Book 23 | Hadith Number 4987

Narrated by Abu Humaid Sa'idi
Abu Humaid Sa'idi reported: I came to Allah's Messenger (may peace be upon him) with a cup of milk from Naqi' which had no cover over it, whereupon he said: Why did you not cover it? - even if you had covered it only with a stick. Abu Humaid said that he had been ordered that water-skins be tied during the night, and the doors be closed during the night.