Sahih Muslim
The Book of Drinks (Kitab Al-Ashriba)
Book Number: 023 / Hadith Number: 4990
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?