Bekhorot 4A

גמ׳ אינהו פטרי?! אדם ⁠— אדם פטר, בהמה ⁠— בהמה פטרה, דכתיב: ״קח את הלוים תחת בכור בבני ישראל ואת בהמת הלוים תחת בהמתם״!

Talmud: In stating: If the priests and Levites rendered exempt the firstborn children and donkeys of the Israelites in the wilderness from being counted firstborns, the Mishnah indicates that the priests and Levites themselves rendered the animals exempt. The Talmud therefore asks: Did they render the firstborn children and donkeys exempt? With regard to a person, i.e., the Israelite firstborn, the person, i.e., the priests and Levites, rendered them exempt. But with regard to an animal, i.e., the firstborn donkeys of the Israelites, the animal, i.e., the sheep of the priests and Levites, rendered them exempt, as it is written: “Take the Levites in exchange for all the firstborn among the children of Israel, and the animal of the Levites in exchange for their animals” (Numbers 3:45).

אמר אביי, הכי קאמר: כהנים ולוים פטורין בהמתם מקל וחומר ⁠— אם הפקיעה בהמתם של לוים בהמה של ישראל במדבר, דין הוא שתפקיע את של עצמן.

The Talmud answers: Abaye said this is what the Mishnah is saying: The firstborn animals of priests and Levites are exempt from firstborn status, and that is derived from an a fortiori inference: If the animals of the Levites, i.e., the sheep of the priests and Levites, rendered the firstborn status of the animals of the Israelites in the wilderness abrogated, it is only logical that the sheep of the priests and Levites should render the firstborn status of the priests’ and Levites’ own firstborn donkeys abrogated.

אמר ליה רבא: והא ״פטרו״ אינהו קתני!

Rava said to him: How can you interpret the a fortiori inference in the Mishnah as referring to the animals of the priests and Levites? But doesn’t it teach: They, i.e., the priests and Levites, rendered the firstborn children and donkeys of the Israelites exempt? The reference is clearly to the priests and Levites themselves, not their animals.