הלכה על במדבר 18:17

LAWS OF THE FIRST-BORN.
These comprise five precepts, of which two are affirmative precepts and three are negative precepts. Their detailed enumeration is as follows: 1) to set apart the first-born (of the cattle); 2) not to eat an unblemished first-born outside Jerusalem; 3) not to redeem the first-born (of the clean cattle); 4) to set apart tithe of the young of the clean cattle; 5) not to redeem the tithe of cattle. I have included the laws of tithe of cattle with those of first-born of cattle because the procedure is the same in both, and Scripture includes the former with the latter in the text "Thou shalt sprinkle their blood (on the altar)" (Num. 18:17), which text has been traditionally interpreted to refer to the blood of the tithe of cattle and the blood of firstlings.

To not redeem the first-born of a pure animal: That we should not redeem the first-born of a pure (kosher) animal. Since the Torah had commanded that the first-born donkey be redeemed, it may have come to our minds to also redeem the first-born of a pure animal with another animal. Therefore this prevention came to us, that we should never redeem it. And even if one transgressed and redeemed it, it is not redeemed. About this is it stated (Numbers 18:17), "But the first-born of an ox, the first-born of a sheep, or the first-born of a goat should not be redeemed; they are sacred." The verse elucidated the three types of pure (domesticated) beasts that are subject to the commandment of the first-born, as the seven varieties of wild animals are not included in the law of firstborns, and as I explained in the Order of Bo (Sefer HaChinukh 17).