Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 62
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 62, with English translation by Maimonides (Rambam).
שלא ישבע לשוא, שנאמר "לֹא תִשָּׂא אֶת שָׁם ה' אֱלֹהֶיךָ לַשָּׁוְא" (שמות כ, ו; דברים ה, י).
Not to take an oath in vain, as [Exodus 20:7] states: "Do not take the name of God, your Lord, in vain."