Numbers 8:17-19

17Every firstborn son among the Israelites is mine. And all the firstborn male animals are mine too. On the day that I killed all the firstborn sons in Egypt I made them special for myself.

18But now I have not taken all the firstborn sons among the Israelites. I have taken the Levites instead.

19And I have given the Levites as a gift to Aaron and to his sons from among the Israelites. They will work at the Tent of Meeting on behalf of the Israelites. And they will make peace with me on behalf of the Israelites. So there will not be a bad disease among the Israelites when they go near to God's Holy Tent.’