Deuteronômio 12:6 Midrash: Mekhilta DeRabbi Yishmael, Midrash Tanchuma, Sifrei Devarim & Sifrei Bamidbar

Because it is written (Devarim 12:6) "And you shall bring there your burnt-offerings and your sacrifices … and the first-born of your herds and flocks," I might think that he must bring them (the first-born) to the Temple; it is, therefore, written (Exodus 13:2) "in man and beast, he (the first-born) is mine." The first-born of a beast is likened to the first-born of a human being, and the first-born of a human being, to the first-born of a beast.

A firstborn human must be redeemed when thirty days old;¹³Firstborn boys are redeemed through the pidyon ha-ben ceremony at the end of thirty days. As indicated above, those born by Caesarean section are exempt. if it survives less than this it is considered a premature child and is exempt from this regulation. The firstborn beast must be redeemed on the eighth day; if it survives less than this it is considered a premature birth. In reference to a human, it is written: And their redemption money—from a month old shalt thou redeem them (Num. 18:16), while in regard to beasts, it is written: But from the eighth day and henceforth it may be accepted (Lev. 22:27). That is, after one is able to lead it to the Temple, since it is said: And thither you shall bring your burnt offerings (Deut. 12:6). In the case of the firstborn of your flock and herds, Scripture says: Thou shalt redeem. This implies that one may redeem the offering from the priest whenever (he wishes). (But if that is so)¹⁴Etz Joseph omits the parenthesized words. Why does the Scripture say: Thou shalt sanctify to the Lord? So that you receive a reward for so doing. But even if you should not sanctify it, it is consecrated, nevertheless, to the Lord, since as Scripture says: It is mine. Why then does Scripture decree Thou shalt sanctify it? In order that you may be rewarded for doing so (voluntarily).

Variantly: The bechor of a man is likened to the bechor of a beast, and the bechor of a beast to the bechor of a man. Just as with a beast, a miscarriage (of the first pregnancy) exempts (the second fetus) from (the law of) the bechor, so, with men. And just as with the bechor of a man, one may give it to the Cohein wherever he wishes, so, with the bechor of a beast. From (Devarim 12:6) "And you shall bring there (to the Temple) your burnt-offerings and your sacrifices, etc.", I might think that even if he was far away he must bring it (his first-born) to the Temple. It is, therefore, written "in man and beast he is Mine." The bechor of a man is hereby likened to the bechor of a beast. Just as the latter may be given to a Cohein wherever he wishes, so, the former. And just as the bechor of a man must be cared for for thirty days (before being given to the Cohein), so, the bechor of a beast.