Negative commandment · Lo Ta’aseh #62 of 613
Not to take an oath in vain
שלא ישבע לשוא
The verse · Shemot 20:7
לֹא תִשָּׂא אֶת־שֵׁם־ה׳ אֱלֹהֶיךָ לַשָּׁוְא כִּי לֹא יְנַקֶּה ה׳ אֵת אֲשֶׁר־יִשָּׂא אֶת־שְׁמוֹ לַשָּׁוְא
“Thou shalt not take the name of the Lord thy God in vain; for the Lord will not hold him guiltless that takes his name in vain.”
The Rambam’s words
Not to take an oath in vain, as [Exodus 20:7] states: "Do not take the name of G-d, your Lord, in vain."
שלא ישבע לשוא, שנאמר "לֹא תִשָּׂא אֶת שֵׁם ה' אֱלֹהֶיךָ לַשָּׁוְא" (שמות כ, ו; דברים ה, י).
Why · Sefer HaChinuch #30
To not swear in vain
“It is from the roots of this commandment [that it is] for people to know and fix in their souls and strengthen the faith in their hearts about G-d, blessed be He — Who is in the Heavens above and exists forever — that there is nothing else like His existence.”
Who & when
“And [it] is practiced in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Oaths הלכות שבועות