Num Numbers, chapter Book of Numbers, chapter 3
Again the LORD spoke to Moses, saying, “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, for all the firstborn are Mine. On the day I struck down every firstborn in the land of Egypt, I consecrated to Myself all the firstborn in Israel, both man and beast. They are Mine; I am the LORD.”