Eruvin 61B

גמ׳ אמר רב יהודה אמר שמואל: שבת בעיר חריבה, לרבנן מהלך את כולה וחוצה לה אלפים אמה. הניח את עירובו בעיר חריבה ⁠— אין לו ממקום עירובו אלא אלפים אמה. רבי אלעזר אומר: אחד שבת ואחד הניח ⁠— מהלך את כולה וחוצה לה אלפים אמה.

Talmud: Rav Yehuda said that Shmuel said: If one established his Shabbat residence in a desolate city whose walls are still standing, according to the Rabbis he may walk through all of it as though it were 4 cubits, and he may walk an additional 2,000 cubits beyond it. If, however, he merely placed his eiruv in a desolate city, he has only 2,000 cubits from the place of his eiruv. The Rabbis distinguish between one who establishes his Shabbat residence by actually being present in that location at the onset of Shabbat and one who does so by placing his eiruv there before Shabbat. R' Elazar says: Whether he established his Shabbat residence through his physical presence or he merely placed his eiruv there, he may walk through all of it and another 2,000 cubits beyond it.

מיתיבי, אמר להן רבי עקיבא: אי אתם מודים לי בנותן את עירובו במערה, שאין לו ממקום עירובו אלא אלפים אמה? אמרו לו: אימתי, בזמן שאין בה דיורין. הא באין בה דיורין מודו ליה?!

The Talmud raises an objection based upon the Mishnah. R' Akiva said to the Rabbis: Do you not concede to me that one who places his eiruv in a cave has only 2,000 cubits from the place of his eiruv? They said to him: When does this apply? When the cave has no residents. Consequently, when it has no residents the Rabbis concede to R' Akiva that one has only 2,000 cubits from the place of his eiruv. This contradicts R' Elazar’s assertion that, according to the Rabbis, even if one places his eiruv in the abandoned city, he may walk through all of it and another 2,000 cubits beyond it.