Musnad Ahmad 27:664

Khalaf ibn al-Walid narrated to us, Abu Ma'shar narrated to us, from Nafi', from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) passed by some food, and its owner had made it look good. He inserted his hand into it, and behold, it was inferior food. He said: "Sell this separately and this separately. Whoever cheats us is not one of us."