Gittin, Chapter 1, Mishnah 2. In this mishnah Rabbi Yehudah maps out the borders of Eretz Yisrael, and defines what counts as "medinas hayam" - that is, what is considered outside the Land, so that an agent bringing a get from there must declare, "It was written in my presence and signed in my presence."
The borders of Eretz Yisrael according to Rabbi Yehudah:
"MeRekem lamizrach, veRekem kamizrach" - the city of Rekem, already mentioned in the previous mishnah: from Rekem and eastward is considered medinas hayam, and Rekem itself also has the status of the east, meaning it too is medinas hayam.
"Me'Ashkelon ladarom, ve'Ashkelon kadarom" - from Ashkelon and southward is considered medinas hayam, and Ashkelon itself is treated as part of the south: it is not included in Eretz Yisrael, but is medinas hayam.
"Me'Akko latzafon, ve'Akko katzafon" - from Akko and northward is considered medinas hayam, and Akko itself also has the status of the north, outside the borders of Eretz Yisrael.
The opinion of Rabbi Meir:
"Rabbi Meir omer: Akko ke'Eretz Yisrael legittin" - Rabbi Meir disagrees and holds that with regard to gittin Akko has the status of Eretz Yisrael, and it is therefore within the borders.
The mishnah has no need to address the western border at all, since in the west the sea itself serves as the boundary.
In summary: in this mishnah Rabbi Yehudah established three borders separating Eretz Yisrael from medinas hayam - Rekem in the east, Ashkelon in the south and Akko in the north, with all three cities themselves included in medinas hayam; and Rabbi Meir disagrees, holding that Akko has the status of Eretz Yisrael with regard to gittin.