Numbers 8:17-26

17 for Mine [is] every first-born among the sons of Israel, among man and among beast; in the day of my smiting every first-born in the land of Egypt I sanctified them for Myself;
18 and I take the Levites instead of every first-born among the sons of Israel:
19 `And I give the Levites gifts to Aaron and to his sons, from the midst of the sons of Israel, to do the service of the sons of Israel in the tent of meeting, and to make atonement for the sons of Israel, and there is no plague among the sons of Israel in the sons of Israel's drawing nigh unto the sanctuary.'
20 And Moses doth -- Aaron also, and all the company of the sons of Israel -- to the Levites according to all that Jehovah hath commanded Moses concerning the Levites; so have the sons of Israel done to them.
21 And the Levites cleanse themselves, and wash their garments, and Aaron waveth them a wave-offering before Jehovah, and Aaron maketh atonement for them to cleanse them,
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,
25 and from a son of fifty years he doth return from the host of the service, and doth not serve any more,
26 and he hath ministered with his brethren in the tent of meeting, to keep the charge, and doth not do service; thus thou dost to the Levites concerning their charge.'