Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 114
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 114, with English translation by Maimonides (Rambam).
להיות מעריך אדם נותן דמים הקצובין בפרשה, שנאמר: "אִישׁ כִּי יַפְלִא נֶדֶר" (ויקרא כז, ב).
For a person who makes an endowment valuation to give the specific amount of money stated in the [Torah] portion, as [Leviticus 27:2] states: "When a person expresses a vow...."