Eruvin 48A

מחיצה של ברזל להפסיקו. מחייך עליה רבי יוסי ברבי חנינא.

an iron partition to divide it into two separate areas, so that the residents of both places may draw water from it. R' Yosei, son of R' Ḥanina, would laugh at this teaching, as he deemed it unnecessary.

מאי טעמא קא מחייך? אילימא משום דתני לה כרבי יוחנן בן נורי לחומרא, ואיהו סבירא ליה כרבנן לקולא. ומשום דסבר לקולא, מאן דתני לחומרא מחייך עלה?

The Talmud asks: Why did R' Yosei, son of R' Ḥanina, laugh? If you say that it is because R' Ḥiyya taught the baraita stringently, in accordance with the opinion of R' Yoḥanan ben Nuri, saying that ownerless objects acquire a place of residence, and R' Yosei, son of R' Ḥanina holds leniently, in accordance with the opinion of the Rabbis and says that those objects do not acquire residence, this is difficult. Just because he holds leniently, does he laugh at one who teaches stringently?

אלא, משום דתניא: נהרות המושכין ומעיינות הנובעין ⁠— הרי הן כרגלי כל אדם.

Rather, he must have laughed for a different reason, as it was taught in a baraita: Flowing rivers and streaming springs are like the feet of all people, as the water did not acquire residence in any particular spot. Consequently, one who draws water from rivers and springs may carry it wherever he is permitted to walk, even if it had previously been located outside his Shabbat limit. According to R' Yosei, son of R' Ḥanina, the same halakha should apply to the water in the ditch.

ודילמא במכונסין.

The Talmud rejects this argument: No proof can be brought from this ruling concerning rivers and springs, as perhaps we are dealing here with a ditch of still, collected water that belongs exclusively to the residents of that particular place.