Jews May Not Deceive Others

Chullin 94a. Shmuel said: One may not deceive another person, even a non-Jew. This was not said explicitly by Shmuel, but was derived from the following story: Shmuel once crossed the river using a ferryboat. He told his servant to pay the ferryman. The servant gave the ferryman a non-kosher chicken, allowing the ferryman to assume that it was kosher.

The Talmudic commentator Rashi explains: Shmuel's law explains why the Mishnah says that one may not give a non-Jew a piece of meat from which the sciatic nerve (forbidden to Jews) was not removed. The non-Jew might not notice this and may assume that the Jew is giving him valuable kosher meat. He will then feel gratitude toward the Jew, a gratitude based on a false premise.

This law is codified by Maimonides (Laws of Sale 18:3) and by the Shulchan Aruch (Choshen Mishpat 228:6).