Numbers 8:11

8:11 And Aaron shall offer the Levites before the LORD [for] an offering of the children of Israel, that they may execute the service of the LORD.

Numbers 8:13

8:13 And thou shalt set the Levites before Aaron, and before his sons, and offer them [for] an offering unto the LORD.

Numbers 3:12

3:12 And I, behold, I have taken the Levites from among the children of Israel instead of all the firstborn that openeth the matrix among the children of Israel: therefore the Levites shall be mine;