Sahih Muslim Book 23, Hadith Number 4989.

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

Jabir b ‘Abdullah reported: We were with Allah’s Messenger (may peace be upon him) and lie asked for water. A person said: Allah’s Messenger, may we not give you Nabidh to drink? He (the Holy Prophet) said: Yes (you may). He (the narrator) said: Then that person went out speedily and brought a cup containing Nabidh, whereupon Allah’s Messenger (may peace be upon him) said: Why did you not cover it? – even if it is with a wood. He said that then he drank it.