Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 157
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 157, with English translation by Maimonides (Rambam).
שלא לעבור על דברים שאסר אדם על נפשו, שנאמר "לא יחל, דברו" (במדבר ל,ג).
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."