אורח חיים ת׳Orach Chayim 400

Se’if 1א׳

מִי שֶׁהָיָה בָּא בַּדֶּרֶךְ לִכָּנֵס לָעִיר וְיָשַׁב בַּדֶּרֶךְ לָנוּחַ, וְחָשְׁכָה לוֹ, וְלֹא יָדַע שֶׁהוּא בִּתְחוּם הָעִיר וְאַחַר כָּךְ מָצָא עַצְמוֹ בִּתְחוּמָהּ, קָנָה שְׁבִיתָה בָּעִיר, וְנִכְנָס לְתוֹכָהּ בְּשַׁבָּת, וּמְהַלֵּךְ אֶת כֻּלָּהּ וְחוּצָה לָהּ אַלְפַּיִם אַמָּה לְכָל רוּחַ. אֲבָל אִם לֹא הָיָה דַּעְתּוֹ לִכָּנֵס לִמְדִינָה זוֹ, קָנָה שְׁבִיתָה בִּמְקוֹמוֹ, וּמִשָּׁם יֵשׁ לוֹ אַלְפַּיִם אַמָּה לְכָל רוּחַ. וְאִם כָּלִים אַלְפַּיִם אַמָּה מִמְּקוֹם שְׁבִיתָתוֹ בַּחֲצִי הָעִיר, אֵינוֹ מְהַלֵּךְ בָּעִיר יוֹתֵר מֵחֶצְיָהּ. הַגָּה: וְהוּא הַדִּין אִם הָיָה בְּדַעְתּוֹ לִכָּנֵס לָעִיר וְאָמַר שְׁבִיתָתִי בִּמְקוֹמִי, נָמֵי דִּינָא הָכֵי. (הַמַּגִּיד פֶּרֶק כ"ז).

1. A Traveler Who Sat Down to Rest Without Knowing Whether or Not He is Within the Shabbos Limits: One who was on his way to enter a city and sat down on the way to rest, and darkness fell upon him and he did not realize that he was within the city limits, and later found himself within its limits, acquires a residence in the city and may traverse its entire length and breadth on Shabbat and proceed another 2000 cubits beyond the town in all directions. However, if he had absolutely no intention to enter this town and acquired a residence there instead, {i.e. the place in which he was located at the onset of Shabbos is established as his Shabbos place.} he may traverse a distance of two thousand cubits in any direction from his residence. If the 2000 [cubits] from his Shabbos place conclude in the middle of the town, he may not walk in the city beyond that point. Rema: The same law applies if he had intended to enter the city and said, "My residence is in my place," as well. (The Magid, Chapter 27)