Reference for Eruvin 5:6
עִיר שֶׁל יָחִיד וְנַעֲשֵׂית שֶׁל רַבִּים, מְעָרְבִין אֶת כֻּלָּהּ. וְשֶׁל רַבִּים וְנַעֲשֵׂית שֶׁל יָחִיד, אֵין מְעָרְבִין אֶת כֻּלָּהּ, אֶלָּא אִם כֵּן עָשָׂה חוּצָה לָהּ כְּעִיר חֲדָשָׁה שֶׁבִּיהוּדָה, שֶׁיֵּשׁ בָּהּ חֲמִשִּׁים דִּיוּרִים, דִּבְרֵי רַבִּי יְהוּדָה. רַבִּי שִׁמְעוֹן אוֹמֵר, שָׁלֹשׁ חֲצֵרוֹת שֶׁל שְׁנֵי בָתִּים:
The city of an individual, [as when one man acquired all of it and rented out all of its houses to others], which then became a public city — eruvin are made for the whole, [as when it belonged to the individual, no (eruv-less) "remnant" being provided]. But a public city which became the city of an individual — eruvin are not made for the whole. [For it is forbidden to make eruvin for a public city without leaving particular houses without an eruv to serve as a sign that (carrying between the others is permitted) by reason of eruv, so that the ordinance of the public domain not be forgotten. And this city, since it was once a public city and required a "remnant," even though it now belongs to an individual, the original procedure is followed.], unless it made (a city) outside it [a "remnant," not making an eruv between it and the rest of the city. We are hereby apprised that even a remnant "outside it" avails for the rest of the city.] as the city, Chadashah, in Judah, where there were [only] fifty dwellers. [This was the smallest city in the entire land of Judah, and it served as a "remnant" for a large city next to it, being of the size required to serve as an eruv-less "remnant" for the (large) city.] These are the words of R. Yehudah. R. Shimon says: Three courtyards of two houses each (constitute a "remnant.") [The halachah is that even one house in one courtyard constitutes a "remnant." And a city which has only one entrance, even a public city, does not require a "remnant."]