Midrash su Levitico 27:33

4) R. Shimon said: Was not ma'aser (a tithe) was in the category (of all consecrated animals in respect to substitution). Why, then, was it singled out for special mention? (viz. Vayikra 27:33) To teach: Just as ma'aser is an individual (as opposed to a communal) offering, so all (such animals) are included (in respect to substitution), and just as ma'aser is an altar offering, so all (such animals are included).

4) (Vayikra 27:33) ("He shall not discriminate between good and bad, and he shall not substitute for it; and if he did substitute for it, then it and its substitute shall be holy. It shall not be redeemed.") Because it is written (Devarim 12:11) "and all the choicest of your vows," I might think that he should "spy out" (his flock) and pick out the choicest (for ma'aser); it is, therefore, written "he shall not discriminate between good and bad." "and he shall not substitute for it": If he did, he receives forty lashes. "if substitute he shall substitute": to include his wife. "and if substitute he shall substitute": to include his heir. "then it and its substitute shall be holy. It shall not be redeemed": About a bechor it is written (Bamidbar 18:17) "you shall not redeem," (but) it is sold, when whole, alive. And when blemished, (it is sold) alive or slaughtered; but (it is) not (sold) slaughtered when whole. And about ma'aser it is written (here) "It shall not be redeemed." And it is not to be sold neither alive nor slaughtered; nor whole nor blemished.

5) And whence is it derived that it is a mitzvah to tithe a beast? From "shall be holy." R. Yossi Haglili says: It is written here "ya'avor" ("that shall pass"), and, elsewhere (Shemoth 13:12) "veha'avarta" ("And you shall set apart [every firstling of the womb"]) — whereby we are apprised that it is a mitzvah to tithe a beast.