Numbers 4:9

“So when the tabernacle is to set out, the Levites shall take it down; and when the tabernacle encamps, the Levites shall set it up. But the layman who comes near shall be put to death.

But he did not give any to the sons of Kohath because theirs was the service of the holy objects, which they carried on the shoulder.

Then the Kohathites set out, carrying the holy objects; and the tabernacle was set up before their arrival.

And the Levites were numbered from thirty years old and upward, and their number by census of men was 38,000.

“Yet they shall be ministers in My sanctuary, having oversight at the gates of the house and ministering in the house; they shall slaughter the burnt offering and the sacrifice for the people, and they shall stand before them to minister to them.