Substitution: The Levites for the Firstborn of the Israelites
Source: Nav
Numbers 3:12, 41, 45
“Behold, I have taken the Levites from among the children of Israel in place of every firstborn Israelite from the womb. The Levites belong to Me, / You are to take the Levites for Me—I am the LORD—in place of all the firstborn of Israel, and the livestock of the Levites in place of all the firstborn of the livestock of the Israelites.” / “Take the Levites in place of all the firstborn of Israel, and the livestock of the Levites in place of their livestock. The Levites belong to Me; I am the LORD.
Numbers 8:18
But I have taken the Levites in place of all the firstborn among the sons of Israel.