Numbers 4:5
When the camp is; to set forward, Aaron and his sons shall go into the tabernacle of the covenant, and the holy of holies, and shall take down the veil that hangeth before the door, and shall wrap up the ark of the testimony in it,

Numbers 4:15
And when Aaron and his sons have wrapped up the sanctuary and the vessels thereof at the removing of the camp, then shall the sons of Caath enter in to carry the things wrapped up: and they shall not touch the vessels of the sanctuary, lest they die. These are the burdens of the sons of Caath: in the tabernacle of the covenant:

Numbers 7:9
But to the sons of Caath he gave no wagons or oxen: because they serve in the sanctuary and carry their burdens upon their own shoulders.

Deuteronomy 10:8
At that time he separated the tribe of Levi, to carry the ark of the covenant of the Lord, and to stand before him in the ministry, and to bless in his name until this present day.

1 Chronicles 23:27
So according to the last precepts of David, the sons of Levi are to be numbered from twenty years old and upward.

2 Chronicles 35:3
And he spoke to the Levites, by whose instruction all Israel was sanctified to the Lord, saying: Put the ark in the sanctuary of the temple, which Solomon the son of David king of Israel built: for you shall carry it no more: but minister now to the Lord your God, and to his people Israel.