Rashi on Genesis 11:2/רש"י על בראשית י״א:ב׳

1בנסעם מקדם AS THEY JOURNEYED FROM THE EAST where they were then dwelling, as it is written above, (Genesis 10:30) “And their dwelling place was … the mountain of the East”. From there they journeyed to search out for themselves a place that would accommodate them all, but they found none except Shinar (Genesis Rabbah 38:7).