Musnad Ahmad 30:174

Sufyan narrated to us, from al-Ala’, from his father, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) passed by a man selling food. He asked him, "How do you sell?" He informed him, and it was revealed to him: "Insert your hand into it." He inserted his hand into it, and behold, it was wet. The Messenger of Allah (may Allah bless him and grant him peace) then said, "He is not one of us who cheats."