Numbers 2:3, 10

On the east side, toward the rising of the sun, those of the standard of the forces with Judah shall camp according to their armies; and Nahshon the son of Amminadab shall be the leader of the children of Judah.”

“On the south side shall be the standard of the forces with Reuben according to their armies, and the leader of the children of Reuben shall be Elizur the son of Shedeur.”