Positive commandment · Mitzvat Aseh #87 of 613
That an animal substituted for a sacred offering be regarded as sacred
להיות התמורה קודש
The verse · Vayikra 27:33
לֹא יְבַקֵּר בֵּין־טוֹב לָרַע וְלֹא יְמִירֶנּוּ וְאִם־הָמֵר יְמִירֶנּוּ וְהָיָה־הוּא וּתְמוּרָתוֹ יִהְיֶה־קֹּדֶשׁ לֹא יִגָּאֵל
“He shall not search whether it be good or bad, neither shall he change it: and if he change it at all, then both it and its substitute shall be holy; it shall not be redeemed.”
The Rambam’s words
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."
להיות התמורה קודש, שנאמר: "והיה הוא ותמורתו יהיה קודש" (ויקרא כז,י; ויקרא כז,לג).
Why · Sefer HaChinuch #352
One who exchanges the beast of a sacrifice with another beast, such that both of them be consecrated
“It is from the roots of the commandment that G-d, blessed be He, wanted to instill His fear in the hearts of people in all matters of sanctity, and as I wrote in the building of the holy House and its vessels in the Order of Vayikchu Li Terumah (Sefer HaChinukh 95).”
Who & when
“And it is practiced in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Substitution הלכות תמורה