Numbers 8:9-11

And thou shalt bring the Levites before the tabernacle of the congregation: and thou shalt gather the whole assembly of the children of Israel together:

And thou shalt bring the Levites before the LORD: and the children of Israel shall put their hands upon the Levites:

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.