Musnad Ahmad 33:857

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-A'mash, from Abu Sufyan, from Jabir, who said: Abu Humayd al-Ansari came to the Messenger of Allah, may Allah bless him and grant him peace, with a vessel containing milk, carrying it uncovered. The Prophet, may Allah bless him and grant him peace, said to him: "Would that you had covered it, even by placing a stick across it."