Numbers 3:11-13

The LORD spoke to Moses, saying, “Behold, I have taken the Levites from amongst the children of Israel instead of all the firstborn who open the womb amongst the children of Israel; and the Levites shall be mine, for all the firstborn are mine. On the day that I struck down all the firstborn in the land of Egypt I made holy to me all the firstborn in Israel, both man and animal. They shall be mine. I am the LORD.”