Numbers 3:12-15

“Behold, I have taken the Levites from among the sons of Israel instead of and as a substitute for every firstborn, that is, the first that opens the womb among the Israelites. So the Levites shall be Mine, for all the firstborn are Mine. On the day that I killed all the firstborn in the land of Egypt, I consecrated for Myself all the firstborn in Israel, both man and animal. They shall be mine; I am the LORD.” Then the LORD spoke to Moses in the Wilderness of Sinai, saying, “Number the sons of Levi by their fathers’ households, by their families (clans). You shall number every male from a month old and upward.”