פירוש על קידושין 122:2

To R. Haninan b. Gamaliel the words “in the land of Canaan” are necessary to teach that if the tribes of Reuben, Gad and half of Menashe don’t cross over they will still have possession in the land of Canaan. This is essentialy what R. Hanina b. Gamaliel said in the mishnah.

R. Meir responds that we would not have thought that for the words “among you” imply anywhere you live. This leaves the words “in the land of Canaan” extraneous and therefore available for a midrash.