Numbers 1:50
But you shall appoint the Levites over the tabernacle of testimony, and over all the vessels thereof, and over all things that belong to it: they shall bear the tabernacle, and all the vessels thereof; and they shall minister to it, and shall encamp round about the tabernacle.
Torrey's Topical Textbook—AKJV

Numbers 8:24
This is it that belongs to the Levites: from twenty and five years old and upward they shall go in to wait on the service of the tabernacle of the congregation:
Torrey's Topical Textbook—AKJV

Numbers 18:2-4
And your brothers also of the tribe of Levi, the tribe of your father, bring you with you, that they may be joined to you, and minister to you: but you and your sons with you shall minister before the tabernacle of witness.
Torrey's Topical Textbook—AKJV