Chagigah 23A

ונפסקה רצועה של סנדלו, ונטלה והניחה על פי חבית, ונפלה לאויר החבית, ונטמאת. באותה שעה אמרו: הנושא את המדרס ⁠— נושא את התרומה, אבל לא את הקדש.

and the strap of his sandal, which had been rendered ritually impure by being trodden by a zav, broke off, and he picked up the strap and placed it on top of the mouth of the jug, and it fell into the airspace of the jug, thereby rendering the entire jug impure. At that time the rabbis said and issued a permanent decree that one who carries an object that is impure by having been trodden by a zav may carry teruma at the same time, but not sacrificial food.

אי הכי, תרומה נמי! הא מני ⁠— רבי חנניה בן עקביא היא, דאמר: לא אסרו אלא בירדן ובספינה, וכמעשה שהיה.

The Talmud asks: If so, if they prohibited doing so due to that incident, they should have made the same decree with regard to teruma also, because if the same incident would occur with teruma it would defile the teruma as well. The Talmud answers: In accordance with whose opinion is this stated? It is that of R' Ḥananya ben Akavya, who said concerning a similar issue: They prohibited it only in the Jordan River, and only in a boat, in a situation similar to the incident that occurred, which triggered the enactment. In his opinion, whenever the rabbis enacted a decree prohibiting something due to a particular incident, they did not extend the prohibition to related cases, but only to the same set of circumstances that pertained to that specific incident.

מאי היא? דתניא: לא ישא אדם מי חטאת ואפר חטאת ויעבירם בירדן ובספינה, ולא יעמוד בצד זה ויזרקם לצד אחר, ולא ישיטם על פני המים, ולא ירכב על גבי בהמה, ולא על גבי חבירו, אלא אם כן היו רגליו נוגעות בקרקע. אבל מעבירן על גבי הגשר ואינו חושש; אחד הירדן ואחד שאר הנהרות. רבי חנניה בן עקביא אומר: לא אסרו אלא בירדן ובספינה, וכמעשה שהיה.

The Talmud presents the background to R' Ḥananya ben Akavya’s statement: What is that enactment that R' Ḥananya ben Akavya was referring to? As it is taught in a baraita: A person may not carry the water of purification and the ashes of purification and transport them across the Jordan River, and this is if he is on a boat. Nor may he stand on one side of the river and throw them to the other side. Nor may he float them across the river. Nor may he ford the river riding on an animal or on his friend and carrying the water or ashes of purification, unless his feet are touching the ground as he fords the river. But he may transfer them across the river over a bridge without concern about transferring them improperly. This decree applies both to the Jordan and to other rivers. R' Ḥananya ben Akavya says: The rabbis prohibited these acts only in the Jordan River, and only if he transports them in a boat, and in circumstances exactly like those of the incident that occurred.