Rashi on Numbers 2:17/רש"י על במדבר ב׳:י״ז

1ונסע אהל מועד AND THE TENT OF MEETING SHALL SET FORWARD after these two divisions (those of Judah and Reuben), as is shown by the position of this statement in the narrative.

2כאשר יחנו כן יסעו AS THEY ENCAMP SO SHALL THEY JOURNEY — just as I have explained (v. 9): their positions when marching shall be the same as when encamping — each division marches on that side that is assigned to it.

3על ידו means, in his place. The term יד does not even in this sense lose its original meaning, viz., that of “hand’’ — the area at one’s side is termed על ידו “that which is by his hand”, being that which is closest to him, and always within the reach of his hand (and not beyond it); en son aise in O. F.