Numbers 2:3, 10

And those that encamp on the east side toward the sunrising shall be they of the standard of the camp of Judah, according to their hosts: and the prince of the children of Judah shall be Nahshon the son of Amminadab.

On the south side shall be the standard of the camp of Reuben according to their hosts: and the prince of the children of Reuben shall be Elizur the son of Shedeur.