Negative commandment · Lo Ta’aseh #157 of 613
Not to violate a vow which a person makes forbidding his use of anything
שלא לעבור על דברים שאסר אדם על נפשו
The verse · Bamidbar 30:3
אִישׁ כִּי־יִדֹּר נֶדֶר לַה׳ אוֹ־הִשָּׁבַע שְׁבֻעָה לֶאְסֹר אִסָּר עַל־נַפְשׁוֹ לֹא יַחֵל דְּבָרוֹ כְּכׇל־הַיֹּצֵא מִפִּיו יַעֲשֶׂה
“If a man vow a vow to the Lord, or swear an oath to bind his soul with a bond; he shall not break his word, he shall do according to all that proceeds out of his mouth.”
The Rambam’s words
Not to violate a vow which a person makes forbidding his use of anything, as [Numbers 30:3] states: "He shall not violate his word."
שלא לעבור על דברים שאסר אדם על נפשו, שנאמר "לא יחל, דברו" (במדבר ל,ג).
Why · Sefer HaChinuch #407
That we not profane our words from vows
“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 this commandment is practiced in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Oaths הלכות שבועות