Avodah Zarah 72B

אם יש בו עכבת יין ⁠— אסור. הא עכבת יין במאי קא מתסרא? לאו בנצוק? שמע מינה: נצוק חיבור.

if there is a remnant of wine left in the funnel, it is rendered forbidden. In what manner is this remnant of wine rendered forbidden? Is it not by the stream of wine going down from the funnel into the non-Jew’s vessel, thereby connecting the non-Jew’s vessel and the funnel and rendering the wine in the funnel forbidden? Conclude from it that a stream constitutes a connection.

תני רבי חייא: שפחסתו צלוחיתו, אבל לא פחסתו צלוחיתו ⁠— מאי? לא, תפשוט דנצוק אינו חיבור! לא, פחסתו צלוחיתו ⁠— תפשוט לך דאסור, נצוק ⁠— תיבעי.

R' Ḥiyya teaches that the Mishnah is referring to a situation where the wine in the non-Jew’s jug rose and reached the funnel. The Talmud infers: But in a case where the wine in the jug did not rise and reach the funnel, what is the halakha? It is not forbidden. Therefore, resolve the dilemma and conclude that a stream is not a connection. The Talmud rejects this inference: No, R' Ḥiyya means to say that in a case where the wine in the jug rose and reached the funnel you can resolve the dilemma and conclude that it is forbidden, but with regard to a stream the dilemma remains unresolved.

תא שמע: המערה מכלי לכלי, את שמערה ממנו ⁠— מותר, הא דביני ביני ⁠— אסור. שמע מינה: נצוק חיבור.

The Talmud suggests: Come and hear a proof from the second topic discussed in the Mishnah: In the case of one who pours wine from one vessel into another vessel, the wine left in the vessel from which he poured is permitted. The Talmud infers: But the stream of wine between the two vessels is forbidden. Conclude from it that a stream constitutes a connection.

אי נצוק חיבור, אפילו דגויה דמנא נמי ליתסר! הא לא קשיא, דקא מקטיף קטופי. מכל מקום, נצוק חיבור!

The Talmud raises an objection to this proof: If a stream constitutes a connection, even the wine inside the vessel being poured from should also be forbidden. The Talmud responds to this objection: This is not difficult. It is the case where the one pouring the wine interrupts the flow, so the reason the wine in the vessel being poured from is permitted is that the stream is interrupted; but in any case, a stream is considered a connection.