Musnad Ahmad 658:106

Muhammad and Hashim narrated to us, they said: Shu'bah narrated to us, from Abu Ishaq, from Al-Bara' ibn 'Azib, who said: "We obtained donkeys on the day of Khaybar, so the herald of the Messenger of Allah (peace and blessings of Allah be upon him) called out: 'Overturn the cooking pots.'"