Musnad Abu Ya'la al-Mawsili 108:7

Ibrahim narrated to us, Abd al-Aziz ibn Muslim narrated to us, from al-A’mash, from Abu Sufyan, from Jabir, and from al-A’mash, from Abu Salih, from Abu Hurayrah: That a man who was called Abu Humayd came to the Prophet, may Allah bless him and grant him peace, with a vessel containing milk from al-Naqi’ during the day. The Prophet, may Allah bless him and grant him peace, said: "Would you not have covered it, even if by placing a stick across it?"