Megillah, Chapter Two, Mishnah 3. This mishnah deals with the law of someone who travels on Purim from a city that is not surrounded by a wall to a walled city, or the reverse, and it then moves on to a further question: how much of the Megillah must a person read in order to fulfill his obligation.
The text of the Mishnah:
"Ben ir shehalach lakerach, uven kerach shehalach la'ir - im asid lachzor limkomo, korei kimkomo; ve'im lav, korei imahen".
"Ben ir" - someone who lives in a city that is not surrounded by a wall, which celebrates and reads the Megillah on the fourteenth of Adar.
"Shehalach lakerach" - a kerach is a walled city, which reads on the fifteenth.
"Uven kerach shehalach la'ir" - someone who lives in a walled city and reads on the fifteenth, and went to a city that is not surrounded by a wall, which reads on the fourteenth.
"Im asid lachzor limkomo - korei kimkomo" - if he intends to return to the place he came from, he reads according to the law of his own city: a person of the fourteenth reads on the fourteenth, and a person of the fifteenth reads on the fifteenth.
"Ve'im lav - korei imahen" - if he does not intend to return, he reads together with the residents of the city in which he is staying and follows their law.
What does "intends to return to his place" mean?
The Gemara clarifies the definition of returning to his place. In the case of a resident of a walled city, who reads on the fifteenth, who went to a city that is not surrounded by a wall and is there on the fourteenth: if he intends to return to the walled city on the night of the fourteenth, he does not read with the residents of that city on the night of the fourteenth, but rather reads according to the law of a person of the fifteenth, since on the night of the fourteenth he returns to his place, which reads on the fifteenth. However, if he does not intend to return on the fourteenth, and it turns out that he will be staying in the unwalled city on the day of the fourteenth, even for part of the day only, he reads on the fourteenth.
The basis for this is derived from a verse in the Megillah: "paruz ben yomo" - a "paruz" is one who dwells in a city that is not surrounded by a wall, and even if he stayed there for only one day, he is called a paruz and is considered a resident of an unwalled city, and therefore he reads on the fourteenth.
The opposite case - a dispute among the Rishonim:
The Rishonim discuss the opposite case: a resident of a city that is not surrounded by a wall who is staying in a walled city - by when must he return in order to be considered a resident of his original city? On this there is a dispute:
Some hold that if he intends to return on the night of the fifteenth, he must read on the fourteenth even though he is in a walled city; and if he will be in the walled city on the day of the fifteenth, his status is that of a person of the fifteenth and he must read with them on the fifteenth - in the same manner explained above, that everything is determined according to the fourteenth.
Others understand that in this case too everything depends on the night of the fourteenth: if he will be staying in the walled city at any time, even on the day of the fourteenth, he must read on the fifteenth. He is considered a person of the fifteenth by virtue of the fact that he was not in an unwalled city on the night of the fourteenth, nor for any part of its day.
From where does one read the Megillah?
From here the mishnah moves to an entirely different subject: from where must one read in the Megillah in order to fulfill the obligation? The Tannaim disputed this:
Rabbi Meir says: all of it - the entire Megillah, and this is the practice in actuality.
Rabbi Yehudah says: "me'ish Yehudi" - from the account of Mordechai onward, at the place where Mordechai is introduced toward the beginning of the second chapter.
Rabbi Yosei says: from the account of Haman onward.
In summary: in this mishnah we learned the law of one who travels on Purim between a walled city and an unwalled city - that the ruling depends on whether he intends to return to his place; the Gemara's explanation of the definition of returning, based on the law of "paruz ben yomo"; the dispute among the Rishonim in the opposite case; and the dispute among the Tannaim as to from where one reads the Megillah - Rabbi Meir: all of it, Rabbi Yehudah: from "ish Yehudi", and Rabbi Yosei: from the account of Haman.