Numbers 3:12,13,40-45
And I, behold, I have taken the Levites from among the children of Israel instead of all the firstborn that opens the matrix among the children of Israel: therefore the Levites shall be mine;
Torrey's Topical Textbook—AKJV

Numbers 8:16-18
For they are wholly given to 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 to me.
Torrey's Topical Textbook—AKJV