Musnad Ahmad 33:26

'Abd al-Razzaq narrated to us, Sufyan informed us, and Abu Nu'aym said: Sufyan narrated to us, from Abu al-Zubayr, from Jabir ibn 'Abd Allah, who said: Abu Humayd al-Ansari came with a vessel of milk during the day to the Prophet, peace and blessings of Allah be upon him, while he was at al-Baqi'. The Prophet, peace and blessings of Allah be upon him, said: "Would that you had covered it, even if you had placed a stick across it."