Sahih Muslim Book 23, Hadith Number 4990.

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

Jabir reported that a person who was known as Abu Humaid brought for him (the Holy Prophet) a cup of milk from al-Naqi’. Allah’s Messenger (may peace be upon him) said to him: Why did you not cover it even with a wood across it?