משנה אויף אסתר 2:1

A resident of a town who has gone to a walled city or a resident of a walled city who has gone to a town, if he is to return to his own place he reads according to the rule of his own place, and if not reads with them. From where does a man read the Megillah and thereby fulfill his obligation? Rabbi Meir says: all of it. Rabbi Judah says: from “There was a Jew” (Esther 2:5). Rabbi Yose says: from “After these things” (Esther 3:1).