Positive commandment · Mitzvat Aseh #94 of 613
To fulfill what has come out of his lips, as he vowed
לקיים אדם כל מה שהוציא בשפתיו, מקרבן או צדקה וכיוצא בהם
The verse · Devarim 23:24
מוֹצָא שְׂפָתֶיךָ תִּשְׁמֹר וְעָשִׂיתָ כַּאֲשֶׁר נָדַרְתָּ לַה׳ אֱלֹהֶיךָ נְדָבָה אֲשֶׁר דִּבַּרְתָּ בְּפִיךָ
“That which is gone out of thy lips thou shalt keep and perform; according as thou hast vowed of thy freewill to the Lord thy God, which thou hast promised with thy mouth.”
The Rambam’s words
For a person to fulfill any [promise] which he utters, be it a sacrifice, [a gift to] charity, or the like, as [Deuteronomy 23:24] states: "What you have spoken, take heed to fulfill...."
לקיים אדם כל מה שהוציא בשפתיו, מקרבן או צדקה וכיוצא בהם, שנאמר: "מוצא שפתיך, תשמור ועשית" (דברים כג,כד).
Why · Sefer HaChinuch #575
To fulfill what has come out of his lips, as he vowed
“It is from the roots of the commandment [that it is] because it is not fitting for a person to be lazy about that which he has vowed to do for a commandment, as is well-known among people.”
Who & when
“And this commandment is practiced in every place and at all times, by males and females.”
Where in the Mishneh Torah
Hilchot Vows הלכות נדרים