קַ֣ח אֶת־הַלְוִיִּ֗ם תַּ֤חַת כָּל־בְּכוֹר֙ בִּבְנֵ֣י יִשְׂרָאֵ֔ל וְאֶת־בֶּהֱמַ֥ת הַלְוִיִּ֖ם תַּ֣חַת בְּהֶמְתָּ֑ם וְהָיוּ־לִ֥י הַלְוִיִּ֖ם אֲנִ֥י יְהוָֽה׃
"Prends les Lévites à la place de tous les premiers-nés des enfants d’Israël, et le bétail des Lévites à la place de leur bétail, les Lévites devant m’appartenir, à moi l’Éternel.
English Explanation of Mishnah Bekhorot
Priests and levites are exempt through [an argument made by a] kal vehomer: if they exempted the first-born belonging to the Israelites in the wilderness, it follows all the more so that they should exempt their own. In Numbers 3:45 God instructs Moses to take the Levites in place of the first-born and to take the cattle of the Levites in place of the cattle of the first-born. From here the mishnah derives a kal vehomer (a fortiori) argument. If the Levites exempted the cattle of the first-born in the desert, then their own first-born animals are all the more so exempt from the law of the bekhor. Thus animals owned by priests and Levites are exempt from these laws. As we shall see, the same holds for children of priests and Levites.
Chizkuni
בבניך תפדה, “as well as amongst your sons, you have to redeem.” This law did not become applicable until after the Levites had become sanctified in lieu of the firstborn sons during the second year of the Israelites in the desert. (Numbers 3,45)
Rav Hirsch
אדם ובהמה :כל ,בכור כל wie V. 17; vergl. Kap. 3, 45.