Musnad Abu Ya'la al-Mawsili 29:1

Uthman ibn Abi Shaybah narrated to us, Mu'awiyah ibn Maysarah ibn Shurayh narrated to us, al-Hakam narrated to us, from Qays ibn Abi Gharzah, who said: The Prophet, peace and blessings of Allah be upon him, passed by an owner of food who was selling his food. The Messenger of Allah, peace and blessings of Allah be upon him, said: "O owner of the food, is the bottom of the food like its top?" He said: "Yes." The Messenger of Allah, peace and blessings of Allah be upon him, said: "Whoever cheats the Muslims is not one of them."