Zevachim 37B

לימד על בכור בעל מום, שניתן לכהן. שלא מצינו לו בכל התורה כולה.

This verse teaches that a blemished firstborn is given to a priest as a gift, and that he may eat its meat. This is a novel concept, as we have not found in the entire Torah another halakha similar to it, where a blemished offering is given to the priests for consumption.

ורבי ישמעאל ⁠– נפקא ליה מ״לך יהיה״ דסיפא.

The Talmud asks: And according to R' Yishmael, who interprets this verse differently, from where does he derive this halakha that a blemished firstborn is given to a priest? The Talmud answers: He derives it from the phrase: “As the breast of waving and as the right thigh, it should be yours” (Numbers 18:18), which is the latter clause of that same verse. The repetition of the phrase “should be yours” serves to teach that even the meat of a blemished firstborn should be eaten by the priests.

בשלמא לרבי יוסי הגלילי, דמוקי ליה נמי במעשר ופסח; היינו דכתיב: ״לא תפדה כי קדש הם״⁠ – הם קריבין, ואין תמורתן קריבה.

§ The Talmud raises another question: Granted, according to the opinion of R' Yosei HaGelili, who interprets the verse as referring also to an animal tithe offering and a Paschal offering, this is as it is written in the same verse: “You should not redeem; they are sacred” (Numbers 18:17). The word “they” indicates that only they are sacrificed on the altar, but their substitutes are not sacrificed. In general, if one substitutes a non-sacred animal for one designated as an offering, both the original and the substitute are considered to be consecrated, and they are therefore sacrificed. But if one substitutes a non-sacred animal for a firstborn, an animal tithe offering, or a Paschal offering, the substitute is not sacrificed.