Numbers 4:3-5

Young's Literal Translation

³ from a son of thirty years and upward, even till a son of fifty years, every one going in to the host, to do work in the tent of meeting.

⁴ `This [is] the service of the sons of Kohath in the tent of meeting, the holy of holies:

⁵ that Aaron and his sons have come in, in the journeying of the camp, and have taken down the vail of the hanging, and have covered with it the ark of the testimony;