Musnad Ahmad 775:6

Sufyan narrated to us, from al-Shaybani, from Ibn Abi Awfa, who said: We obtained donkeys outside of the village, and the Messenger of Allah (peace and blessings of Allah be upon him) said: "Overturn the cooking pots with what is in them." I mentioned that to Sa'id ibn Jubayr, and he said: "He only forbade them because they used to eat filth."