Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 106
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 106, with English translation by Maimonides (Rambam).
שלא להמיר את הקודשים, שנאמר "לא יחליפנו, ולא ימיר אותו" (ויקרא כז,י).
Not to substitute [another animal] for [one selected as] a sacred offering, as [Leviticus 27:10] states: "Do not exchange it or substitute another for it."