Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 87
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 87, with English translation by Maimonides (Rambam).
להיות התמורה קודש, שנאמר: "והיה הוא ותמורתו יהיה קודש" (ויקרא כז,י; ויקרא כז,לג).
That an animal substituted for a sacred offering be regarded as sacred, as [Leviticus 27:33] states: "Both [the original animal] and the one substituted for it shall be consecrated."