Numbers 3:8-12

They shall also take care of all the furnishings of the tent of meeting, along with the duties of the sons of Israel, to do the service of the tabernacle.

So you shall assign the Levites to Aaron and to his sons; they are exclusively assigned to him from the sons of Israel.

So you shall appoint Aaron and his sons that they may keep their priesthood, but the layman who comes near shall be put to death.”

Again the Lord spoke to Moses, saying,

“Now, behold, I have taken the Levites from among the sons of Israel instead of every firstborn, the firstborn of the womb among the sons of Israel. So the Levites shall be Mine.