Eruvin 36B

דאי סלקא דעתך תחילת היום קונה עירוב, אי אמר עירבו לי בזה, אמאי לא אמר כלום?

As, if it should enter your mind that an eiruv acquires one’s Shabbat residence at the beginning of the day of Shabbat, then if he said: Establish an eiruv for me with the produce in this flask, why hasn’t he said anything? After nightfall, when Shabbat begins, the flask is already pure, and therefore the teruma of the tithe inside it is also pure and is suitable for an eiruv.

אמר רב פפא: אפילו תימא תחילת היום קונה עירוב, בעינן סעודה הראויה מבעוד יום, וליכא.

Rav Pappa said: This is no proof; even if you say that an eiruv acquires one’s Shabbat residence at the beginning of the day of Shabbat, nonetheless, we require a meal that is fit to be eaten while it is still day, prior to the onset of Shabbat, in order for the eiruv to be valid, and there is none in this case. While it was still day, it was certainly prohibited to consume the contents of the flask, which were still tevel, and therefore it could not be used as an eiruv.

מתני׳ מתנה אדם על עירובו, ואומר: אם באו גוים מן המזרח ⁠— עירובי למערב. מן המערב ⁠— עירובי למזרח. אם באו לכאן ולכאן ⁠— למקום שארצה אלך. לא באו לא לכאן ולא לכאן ⁠— הריני כבני עירי.

Mishnah: A person may make a condition with regard to his eiruv of Shabbat borders. In other words, he need not decide in advance in which direction his eiruv should take effect. For example: he may deposit an eiruv on each of two opposite sides of his town, and say: If non-Jews come from the east, my eiruv is in the west, so that I can escape in that direction; and if they come from the west, my eiruv is in the east. If they come from here and from there, i.e., from both directions, I will go wherever I wish, and my eiruv will retroactively take effect in that direction; and if they do not come at all, neither from here nor from there, I will be like the rest of the inhabitants of my town and give up both eiruvin that I deposited, leaving me with 2,000 cubits in all directions from the town.