Reference for Eruvin 4:8
אִם אֵינוֹ מַכִּיר, אוֹ שֶׁאֵינוֹ בָקִי בַהֲלָכָה, וְאָמַר, שְׁבִיתָתִי בִמְקוֹמִי, זָכָה לוֹ מְקוֹמוֹ אַלְפַּיִם אַמָּה לְכָל רוּחַ. עֲגֻלּוֹת, דִּבְרֵי רַבִּי חֲנִינָא בֶן אַנְטִיגְנוֹס. וַחֲכָמִים אוֹמְרִים, מְרֻבָּעוֹת, כְּטַבְלָא מְרֻבַּעַת, כְּדֵי שֶׁיְּהֵא נִשְׂכָּר לַזָּוִיּוֹת:
If he does not recognize [a tree or a fence] or if he is not expert in the halachah [and does not know that it will avail him to say: "My habitation is in that place"] and he says: "My habitation is in my place" (i.e., wherever he finds himself), his place effects habitation for him, two thousand cubits in a circle on all sides, [for in respect to the cities of refuge from which the two thousand cubits of the tchum Shabbath are derived it is written (Numbers 35:5): "This shall be for them the open spaces of the cities" — For this (i.e., for the cities of refuge) you add the corners (around the circle), but you do not add corners for those acquiring habitation on Shabbath.] These are the words of R. Chanina b. Antignos. The sages say: (two thousand cubits in a) square, as in a square tablet, so that the corners be gained. [The rabbis expound "This" (above) as inclusive, i.e., As this shall be (the ruling for) all who acquire habitation on Shabbath. The halachah is in accordance with the sages. ("as in a square tablet":) Rambam explains that because it is impossible to make an exact square, it is taught: "as in a square tablet." That is, in the manner that men are wont to square, though it not be exact. ("so that the corners be gained":) the four corners of the diagonal, which are an addition to the circle in the midst of the square.]