Shabbat 41B

והלא מצרף? רבי שמעון היא, דאמר: דבר שאין מתכוין ⁠— מותר. מתקיף לה אביי: מידי מיחם שפינה ממנו מים קתני?! מיחם שפינהו קתני!

The Talmud questions this leniency: By putting cold water into the urn, doesn’t it harden the vessel? Cold water poured into a heated metal vessel reinforces the vessel. It is one of the stages in the labor of a blacksmith. How is it permitted to do something similar on Shabbat? The Talmud answers: This Mishnah is in accordance with the opinion of R' Shimon, who stated a principle with regard to the laws of Shabbat: An unintentional act, i.e., a permitted action from which a prohibited labor inadvertently ensues, is permitted. Here too, his intention was to warm the water, not to reinforce the vessel. Abaye strongly objects to this explanation: Does it say in the Mishnah: An urn from which water was emptied? That would indicate that he sought to fill the vessel with other water and warm up that water. Rather, an urn that was removed was taught in the Mishnah, meaning that the urn was removed from the fire, not that the water was emptied from it.

אלא אמר אביי, הכי קאמר: המיחם שפינהו ויש בו מים חמין ⁠— לא יתן לתוכו מים מועטין בשביל שיחומו, אבל נותן לתוכו מים מרובים כדי להפשירן. ומיחם שפינה ממנו מים ⁠— לא יתן לתוכו מים כל עיקר, מפני שמצרף. ורבי יהודה היא, דאמר דבר שאין מתכוין אסור.

Rather, Abaye said this is what the Mishnah is saying: An urn that was removed from the fire and contains hot water, one may not place a small quantity of water in it so that the water will become hot; rather, one may place a large quantity of water in it so that the water will become warm. And with regard to an urn from which water was removed; one may not place any water into it because he hardens the vessel by placing cold water into a hot vessel. And, according to this explanation, our Mishnah is in accordance with the opinion of R' Yehuda, who said that an unintentional act from which a prohibited labor inadvertently ensues is prohibited on Shabbat.