Midrasz do Wyjścia 13:12

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.

And thou shalt set apart unto the Lord all that openeth the womb (ibid. 12). Set apart means to put aside, as in the matter of an inheritance. Then ye shall cause his inheritance to be set apart unto his daughter (Num. 27:8). Simeon the son of Azzai said: Why does Scripture mention Thou shalt set apart all that openeth the womb (Exod. 13:12)? Since it states elsewhere: Whatsoever passeth under the rod,¹⁸Word-play on haavarta (“set apart”) and yavaor (“passeth under”). the tenth shall be holy unto Me the Lord (Lev. 27:32). Does this say (I might deduce) that this includes an orphaned animal? Since the word set aside is used in the former verse, just as in the latter verse, one may not sanctify the priests’ share of the offering except during the life of its mother, so in this instance one may not sanctify the priests’ share except during the lifetime of its mother. If this is so, then just as the latter verse refers only to male animals, so the former verse applies only to male animals. Hence, when Scripture says Whatsoever passeth under the rod, it means (to include) male and female. All that openeth the womb indicates that a prematurely born offspring is exempted from the law of the firstborn. The one that is born after the premature offspring is also considered not to be the firstborn.

Which thou hast. This excludes the animals that are still in the embryonic state when sold to a gentile. Is the one who purchases an animal in the embryonic state obligated to consecrate it? Scripture answers this question with the verse All the firstling males that are born of thy flock and thy herd, those thou shalt sanctify unto the Lord (Deut. 15:19). (This tells us no.¹⁹The embryo was conceived before the sale was made.) The males shall be the Lord’s (Exod. 13:12). R. Yosé stated: You learn from this that if an ewe, which had not given birth previously, bears twin males, they both belong to the priest, since it is said: The males shall be the Lord’s. Every firstling of an ass, thou shalt redeem with a lamb (Exod. 13:12); but not with a calf or with a wild beast, or with a ritually slaughtered animal, or with hybrids, or with a koy.²⁰An antelope or bearded deer. The rabbis were in doubt as to whether it is considered a domesticated animal or a wild beast. The firstling of an ass you may redeem, but not with any other animal. What is meant by Thou shalt surely redeem? You may redeem the firstling of an ass with any impure animal only if it is to be sanctified for the purpose of the upkeep of the Temple.