Eruvin 75B

תניא נמי הכי: נתנו עירובן בחיצונה, ושכח אחד, בין מן החיצונה ובין מן הפנימית, ולא עירב ⁠— שתיהן אסורות, נתנו עירובן בפנימית, ושכח אחד מן הפנימית ולא עירב ⁠— שתיהן אסורות. מן החיצונה ולא עירב ⁠— שתיהן אסורות, דברי רבי עקיבא. וחכמים אומרים: בזו, פנימית מותרת וחיצונה אסורה.

That was also taught in a baraita: If they placed their eiruv in the outer courtyard, and one person forgot to contribute to the eiruv, whether he is a resident of the outer courtyard or of the inner one, they are both prohibited. If they put their eiruv in the inner courtyard, and one resident of the inner courtyard forgot to contribute to the eiruv, they are both prohibited. Similarly, if one of the residents of the outer courtyard did not contribute to the eiruv, they are both prohibited. This is the statement of R' Akiva. And the Rabbis disagree and say: In this case, where the eiruv was deposited in the inner courtyard and the person who forgot to contribute to the eiruv was a resident of the outer one, the inner courtyard is permitted and the outer one is prohibited.

אמר ליה רבה בר חנן לאביי: מאי שנא לרבנן דאמרי פנימית מותרת ⁠— משום דאחדא דשא ומשתמשא, לרבי עקיבא נמי: תיחד דשא ותשמש! אמר ליה: עירוב מרגילה.

Rabba bar Ḥanan said to Abaye: What is different according to the Rabbis, who say that the inner courtyard is permitted? It is because the residents of the inner courtyard can shut the door of their courtyard to the members of the outer one and use the inner courtyard on their own. But if so, according to R' Akiva as well, let the residents of the inner courtyard shut the door of their courtyard to the members of the outer one and use their courtyard on their own. Abaye said to him: If the eiruv of the outer courtyard was not placed in the inner courtyard, your argument would be valid. But the fact that the eiruv is deposited in the inner courtyard accustoms the residents of the outer courtyard to enter it.