Numbers 4:5

4:5 When it is time for the camp to journey, Aaron and his sons must come and take down the screening curtain and cover the ark of the testimony with it.

Numbers 4:49

4:49 According to the word of the Lord they were numbered, by the authority of Moses, each according to his service and according to what he was to carry. Thus were they numbered by him, as the Lord had commanded Moses.

Numbers 7:9

7:9 But to the Kohathites he gave none, because the service of the holy things, which they carried on their shoulders, was their responsibility.