Sahih Muslim Book 23, Hadith Number 4987.

Chapter : Command pertaining to the covering of vessels, and tightening (the mouths) of waterskins, etc.

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.