Numbers 3:9-19 CEB

9 You will give the Levites to Aaron and his sons. They have been assigned as a gift to him from the Israelites.
10 You will appoint Aaron and his sons to be responsible for the priesthood. Any other person who approaches will be put to death.
11 The LORD spoke to Moses:
12 I claim the Levites from the Israelites in place of all the oldest males who open an Israelite womb. The Levites are mine
13 because all the oldest males are mine. When I killed all the oldest males in the land of Egypt, I reserved for myself all the oldest males in Israel, both humans and animals. They are mine; I am the LORD.
14 The LORD spoke to Moses in the Sinai desert:
15 Enroll the Levites by their households and their clans. You will enroll all the males over one month old.
16 Moses enrolled them according to the LORD's word as he was commanded.
17 These were Levi's sons by name: Gershon, Kohath, and Merari.
18 These were the names of Gershon's sons by their clans: Libni and Shimei.
19 Kohath's sons by their clans: Amram, Izhar, Hebron, and Uzziel.