Numbers 8:16-18

8:16 For they [are] wholly given unto me from among the children of Israel; instead of such as open every womb, [even instead of] the firstborn of all the children of Israel, have I taken them unto me. 8:17 For all the firstborn of the children of Israel [are] mine, [both] man and beast: on the day that I smote every firstborn in the land of Egypt I sanctified them for myself. 8:18 And I have taken the Levites for all the firstborn of the children of Israel.