Rashi on Numbers 8:9/רש"י על במדבר ח׳:ט׳

1והקהלת את כל עדת AND THOU SHALT GATHER THE WHOLE ASSEMBLY [OF THE CHILDREN OF ISRAEL] TOGETHER — Because the Levites were made, so to speak, an expiatory offering in their stead, let them come and stand at their offering and put their hands upon them (the Levites) as is prescribed in the case of an offering (cf. Leviticus 1:4).