Musar zu Ijow 31:40

But if he does not meticulously examine his deeds very carefully, thorns will sprout instead of wheat. For he will transgress and stumble in the sin of Onaah (fraud) which we were warned against: "And you shall not oppress one another" (Vayikra 25:17) and our sages of blessed memory said: "even to deceive a non-Jew is forbidden" (Chulin 94a). And scripture states: "the remnant of Israel shall not do iniquity nor speak lies, neither shall a deceitful tongue be found in their mouth" (Tzefania 3:13).