Reference for Eruvin 5:7
מִי שֶׁהָיָה בַמִּזְרָח וְאָמַר לִבְנוֹ, עָרֵב לִי בַמַּעֲרָב, בַּמַּעֲרָב וְאָמַר לִבְנוֹ, עָרֵב לִי בַמִּזְרָח, אִם יֵשׁ הֵימֶנּוּ וּלְבֵיתוֹ אַלְפַּיִם אַמּוֹת, וּלְעֵרוּבוֹ יוֹתֵר מִכָּאן, מֻתָּר לְבֵיתוֹ וְאָסוּר לְעֵרוּבוֹ. לְעֵרוּבוֹ אַלְפַּיִם אַמָּה, וּלְבֵיתוֹ יוֹתֵר מִכָּאן, אָסוּר לְבֵיתוֹ וּמֻתָּר לְעֵרוּבוֹ. הַנּוֹתֵן אֶת עֵרוּבוֹ בְעִבּוּרָהּ שֶׁל עִיר, לֹא עָשָׂה וְלֹא כְלוּם. נְתָנוֹ חוּץ לַתְּחוּם, אֲפִלּוּ אַמָּה אַחַת, מַה שֶׁנִּשְׂכָּר הוּא מַפְסִיד:
If one were in the east, and he said to his son: "Make an eruv for me in the west"; or if he were in the west, and he said to his son: "Make an eruv for me in the east" — if there are from him to his house two thousand cubits, and to his eruv more than that, he is permitted to his house and forbidden to his eruv. (If there are ) to his eruv two thousand, and to his house more than that, he is forbidden to his house and permitted to his eruv. [("If one were in the east":) in the field, and Shabbath descended upon him there, and he were more than two thousand cubits from his eruv, his eruv is no eruv. Since he may not walk and take it, his habitation is in his house, standing as he does within the tchum of his house. And it is assumed that he desires his habitation to be in his house when his eruv is not an eruv. ("he is forbidden to his house":) to count two thousand cubits on all sides of his house.] If one places his eruv in the outskirts (ibur) of the city, [i.e., in one of the houses standing within the seventy and a fraction cubits], he has done nothing. [For without an eruv, too, he has two thousand cubits on all sides, and the entire city with its ibur are reckoned as his four cubits.] If he placed it outside the tchum [i.e., outside the ibur of the city (Thus is it explained in the gemara)], what he gains [on one side], he loses [on the opposite side. For he counts two thousand cubits on all sides of the eruv, and if he places it at the end of a thousand (from the city) eastwards, it is found that the two thousand to the east end three thousand cubits from the city, so that he gains one thousand (to the east), and the two thousand to the west end one thousand cubits west of the city, so that he loses one thousand cubits. We are hereby apprised that the city is not included in the two thousand cubits to the west, but all of it is regarded as four cubits. This, only when the two thousand cubits from the eruv towards the city terminate at the end of the city or beyond it; but if they terminate in the middle of the city or anywhere within it, he may go within the city only as far as the termination of the two thousand cubits of the eruv and not more, as taught below.]