The Messenger of Allah ﷺ passed by a man selling food and asked him how he was selling it. The man told him, then it was revealed to him to put his hand into it. He put his hand in and found it to be wet. The Messenger of Allah ﷺ said, "Whoever cheats is not one of us."
حدثنا أحمد بن محمد بن حنبل حدثنا سفيان بن عيينة عن العلاء عن أبيه
أن رسول الله ﷺ مر برجل يبيع طعاما فسأله كيف تبيع فأخبره فأوحي إليه أن أدخل يدك فيه فأدخل يده فيه فإذا هو مبلول فقال رسول الله ﷺ ليس منا من غش
[Its isnad is sahih]. Ibn Majah collected it in the Book of Trade, Chapter on the Prohibition of Cheating, hadith no. 2224, from the narration of Sufyan bin Uyayna, with the same chain. It is also found in Musnad Ahmad, 2/242, and Hakim graded it sahih according to the criteria of Muslim, 2/8, 9, and Dhahabi agreed with him. Its origin is found in Muslim, hadith no. 102.