Rashi on Genesis 48:6/רש"י על בראשית מ״ח:ו׳

1Although the Land was divided according to the number of heads — as it is written, (Numbers 26:54) “To the more numerous thou shalt give a larger inheritance” — and each person had an equal share except those who were first-born sons (and these received a double share), yet only these of Joseph’s sons bore the name of “tribe” when it became a matter of casting lots for the partition of the land according to the number of the tribes (cf. Numbers 26:55), and of appointing princes to the various tribes and of assigning banners to each of them.