Ketubot 6A

או הלכה כרבי שמעון.

Or, is the halakha in accordance with the opinion of R' Shimon, who ruled that one who is destructive in causing a wound on Shabbat is liable if he did so intentionally?

(איתמר:) בבי רב אמרי: רב שרי ושמואל אסר. בנהרדעא אמרי: רב אסר ושמואל שרי. אמר רב נחמן בר יצחק, וסימניך: אלו מקילין לעצמן ואלו מקילין לעצמן.

With regard to the halakha of having sex with one’s virgin bride on Shabbat, it was stated that in the school of Rav they say: Rav permitted doing so and Shmuel prohibited doing so. In Neharde’a, where Shmuel lived, they say: Rav prohibited doing so and Shmuel permitted doing so. Rav Naḥman bar Yitzḥak said: And your mnemonic is: These are lenient with regard to themselves, and those are lenient with regard to themselves. Each attributes the lenient ruling to the local halakhic authority, whose ruling is binding in that locale.

ורב שרי? והאמר רב שימי בר חזקיה משמיה דרב: האי מסוכריא דנזייתא ⁠— אסור להדוקה ביומא טבא!

The Talmud asks: And did Rav permit having sex in those circumstances? But didn’t Rav Shimi bar Ḥizkiyya say in the name of Rav: In the case of this cloth stopper of a jug [nazyata], it is prohibited to insert it tightly in the spout of the jug on a Festival, because in the process liquid will be squeezed from the cloth, and squeezing liquids is prohibited on Shabbat and Festivals. Apparently, Rav prohibits even unintentional actions.

בההוא אפילו רבי שמעון מודה, דאביי ורבא דאמרי תרוייהו: מודה רבי שמעון ב״פסיק רישיה ולא ימות״.

The Talmud answers: In the case of the jug, even R' Shimon concedes, as it is Abaye and Rava who both say: R' Shimon concedes in the case of: Cut off its head and will it not die, i.e., a case that involves inevitable consequences like the decapitation of an animal, that the action is forbidden. Here, the liquid will inevitably be squeezed from the cloth.