Numbers 1-4

And the Lord spoke to Moses, saying, “Behold, I have taken the Levites from among the people of Israel instead of every firstborn who opens the womb among the people of Israel. 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 consecrated for my own all the firstborn in Israel, both of man and of beast. They shall be mine: I am the Lord.” – Numbers 3:11-13

Scripture Reference: Numbers 1-4