Numbers 8:10

8:10 Then you are to bring the Levites before the Lord, and the Israelites are to lay their hands on the Levites; ¹ 

Numbers 8:18

8:18 So I have taken the Levites instead of all the firstborn sons among the Israelites.

¹ sn The consecration ceremony was to be done in full view of the assembled people. In all probability the laying on of the hands was done through representatives of the tribes, and not all the people. This ritual of the imposition of hands showed that the people were taking part in the consecration, and that the Levites represented them in the service of the Lord.