Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 253
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 253, with English translation by Maimonides (Rambam).
שלא להונות את הגר במקח וממכר, שנאמר "וְלֹא תִלְחָצֶנּוּ" (שמות כב, כ).
Not to cheat a convert in business, as [Exodus, ibid.] continues: "...nor oppress him."