Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 14
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 14, with English translation by Maimonides (Rambam).
שלא לישבע בעבודה זרה לעובדיה, ולא משביעין אותן בה, שנאמר "ושם אלוהים אחרים לא תזכירו, לא יישמע על פיך" (שמות כג,יג).
Not to take an oath on a false god [as requested by] one of its worshipers, nor to have one of them take an oath on [their false god], as [Exodus 23:13] states: "And the name of other gods you shall not mention, nor should it be heard from your mouth."