From Sahih Muslim (Hadith 5242):
Abu Humaid Sa'idi reported:I came to Allah's Messenger (ﷺ) 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 waterskins be tied during the night, and the doors be closed during the night
Reference: Muslim 5242