Numbers 3:11-13

3:11 Then the Lord spoke to Moses: 3:12 “Look, I myself have taken the Levites from among the Israelites instead of every firstborn who opens the womb among the Israelites. So the Levites belong to me, 3:13 because all the firstborn are mine. When I destroyed all the firstborn in the land of Egypt, I set apart for myself all the firstborn in Israel, both man and beast. They belong to me. I am the Lord.”

Numbers 3:15

3:15 “Number the Levites by their clans and their families; every male from a month old and upward you are to number.”