Num Numbers, chapter Book of Numbers, chapter 3
And the LORD spoke unto Moses, saying: 'And I, behold, I have taken the Levites from among the children of Israel instead of every first-born that openeth the womb among the children of Israel; and the Levites shall be Mine; for all the first-born are Mine: on the day that I smote all the first-born in the land of Egypt I hallowed unto Me all the first-born in Israel, both man and beast, Mine they shall be: I am the LORD.'