מדרש אויף במדבר 3:13

“Is [Ephraim] a precious son to Me [li]”? Wherever [the term] li is stated, it never changes, not in this world and not in the World to Come. Regarding priests it is written: “They shall serve as priests to Me [li]” (Exodus 40:15). Regarding Levites it is written: “The Levites shall be Mine [li]” (Numbers 8:14). Regarding Israel it is written: “For the children of Israel are Mine [li]” (Leviticus 25:55). Regarding gifts it is written: “They shall take [gifts] for Me [li]” (Exodus 25:2). Regarding the firstborn it is written: “For every firstborn is Mine [li]” (Numbers 3:13). Regarding the Sanhedrin it is written: “Gather to Me [li] seventy men from the elders of Israel” (Numbers 11:16). Regarding the Land of Israel: “For all the land is Mine [li]” (Exodus 19:5). Regarding Jerusalem: “The city that I have chosen for Me [li]” (I Kings 11:36). Regarding the royal House of David: “For I have seen a king for Me [li] among his sons” (I Samuel 16:1). Regarding the Temple: “They shall craft a Sanctuary for Me [li]” (Exodus 25:8). Regarding the altar: “You shall craft an altar of earth for Me [li]” (Exodus 20:21). Regarding offerings: “You shall be diligent to sacrifice to Me [li]” (Numbers 28:2). Regarding the anointing oil: “This shall be oil of sacred anointment for Me [li]” (Exodus 30:31). That is, every place that li is stated, it never changes, not in this world and not in the World to Come.

Speak unto the children of Israel, that they take for Me an offering (ibid., v. 2). Each time the Holy One, blessed be He, says for Me in the Torah, both this world and the world-to-come are included. For example, For the land shall not be sold in perpetuity, for the land is for Me (Lev. 25:23). Here and in the time-to-come. For all the firstborn are for Me (Num. 3:13); that is in this world and in the time-to-come. The Levites are for Me (ibid. 8:14); in this world and in the time-to-come. Israel, as it is written: And ye shall be for Me a kingdom of priests (Exod. 19:6); in this world and the world-to-come. Hence, That they take for Me an offering alludes to the present and the time-to-come.

“In place of every firstborn, first issue of the womb from the children of Israel” – initially the service was performed by the firstborns. Because [the firstborns] sinned with the calf, the Levites merited to take their place because they did not err with the calf. “For every firstborn is Mine…” – just as it says: “Sanctify for Me every firstborn…” (Exodus 13:2), “They shall be Mine, I am the Lord.” Because it says: “I have hereby taken the Levites from among the children of Israel in place of every firstborn,” do I hear that from this day forward the firstborn are no longer sacred? The verse states: “They shall be”; it teaches that they require redemption.¹⁷Even though the firstborns were replaced by the Levites, they are still holy and they must be redeemed by giving five Shekalim to a priest.