Musnad Abu Ya'la al-Mawsili 109:6

Ishaq narrated to us, Jarir narrated to us, from al-A'mash, from Abu Salih and Abu Sufyan, from Jabir, who said: A man called Abu Humayd came with a cup of milk from al-Naqi'. The Messenger of Allah (peace and blessings of Allah be upon him) said to him: "Would you not have covered it, even by placing a stick across it?"