Musnad Ahmad 658:198

Waki’ narrated to us, from Isra’il, from Abu Ishaq, from Al-Bara’ ibn ‘Azib, who said: The Prophet, peace and blessings of Allah be upon him, passed by us on the day of Khaybar, and we had cooked pots. He said: "What is this?" We said: "Donkeys that we have obtained." He said: "Are they wild or domestic?" We said: "Domestic." He said: "Overturn them."