Jamieson, Fausset and Brown on Numbers 8:9-10

9-10. thou shalt gather the whole assembly of the children of Israel together, &c.--As it was plainly impossible that the whole multitude of the Israelites could do this, a select portion of them must be meant. This party, who laid their hands upon the Levites, are supposed by some to have been the first-born, who by that act, transferred their peculiar privilege of acting as God's ministers to the Levitical tribe; and by others, to have been the princes, who thus blessed them. It appears, from this passage, that the imposition of hands was a ceremony used in consecrating persons to holy offices in the ancient, as, from the example of our Lord and His apostles, it has been perpetuated in the Christian Church.