Numbers 8:21

I have taken the Levites instead of all the firstborn among the children of Israel.

I have given the Levites as a gift to Aaron and to his sons from among the children of Israel, to do the service of the children of Israel in the Tent of Meeting, and to make atonement for the children of Israel, so that there will be no plague among the children of Israel when the children of Israel come near to the sanctuary.”

Moses, and Aaron, and all the congregation of the children of Israel did so to the Levites. According to all that the LORD commanded Moses concerning the Levites, so the children of Israel did to them.

After that, the Levites went in to do their service in the Tent of Meeting before Aaron and before his sons: as the LORD had commanded Moses concerning the Levites, so they did to them.

The LORD spoke to Moses, saying,

“This is what is assigned to the Levites: from twenty-five years old and upward they shall go in to wait on the service in the work of the Tent of Meeting;