Numbers 3:11

8 and they have kept all the vessels of the tent of meeting, and the charge of the sons of Israel, to do the service of the tabernacle;

9 and thou hast given the Levites to Aaron and to his sons; they are surely given to him out of the sons of Israel.

10 `And Aaron and his sons thou dost appoint, and they have kept their priesthood, and the stranger who cometh near is put to death.`

12 `And I, lo, I have taken the Levites from the midst of the sons of Israel instead of every first-born opening a womb among the sons of Israel, and the Levites have been Mine;

13 for Mine [is] every first-born, in the day of My smiting every first-born in the land of Egypt I have sanctified to Myself every first-born in Israel, from man unto beast; Mine they are; I [am] Jehovah.`

14 And Jehovah speaketh unto Moses in the wilderness of Sinai, saying,