Rashi on Numbers 2:9/רש"י על במדבר ב׳:ט׳

1ראשנה יסעו THESE SHALL FIRST JOURNEY — When they saw the cloud departing, the priests blew the trumpets and the camp of Judah moved off first, but when marching, they marched retaining the positions in which they always encamped: the Levites and the wagons in the centre (v. 17), the division of Judah in the East (v. 8), that of Reuben in the South, Ephraim’s in the West (v. 19) and Dan’s in the North (v. 25).