Tosefta do Estery 6:1

The obligation of the Megillah [is to read it] from the beginning until the end, the words of Rabbi Meir. Rabbi Yehuda says, from [the words] "A Jewish man" (Esther 2:5). Rabbi Yosei says, from [the words], "After these events" (Esther 3:1). Rabbi Shimon ben Elazar, from "That night" (Esther 6:1). But all agree that [regardless of the point at which one started], the obligation is to complete [the Megillah] until the very end.