Numbers 8:22-24

Young's Literal Translation

²² and afterwards have the Levites gone in to do their service in the tent of meeting, before Aaron and before his sons; as Jehovah hath commanded Moses concerning the Levites, so they have done to them.

²³ And Jehovah speaketh unto Moses, saying,

²⁴ `This [is] that which [is] the Levites': from a son of five and twenty years and upward he doth go in to serve the host in the service of the tent of meeting,