Shabbat 37B

שמע מינה מצטמק ויפה לו מותר! שאני הכא דקטמה. אי הכי מאי למימרא?! הובערה איצטריכא ליה, מהו דתימא כיון דהובערה הדרא לה למילתא קמייתא, קמשמע לן.

Conclude from this halakha that even when it is food that shrivels and improves by remaining on the fire, it is nevertheless permitted to leave it. The food is already completely cooked and there is no concern lest one come to stoke the coals and ignite the fire. R' Oshaya did not distinguish between different types of foods in permitting this. The Talmud rejects this conclusion: Here, it is different because he covered the coals in the stove with ashes, and that is the reason that he is permitted to leave food on the stove. The Talmud asks: If so, what purpose was there to say this halakha? Is it to teach that if the coals are covered with ashes, there is no room for concern? That is obvious. The Talmud answers: The case where he covered it with ashes and it reignited on Shabbat was necessary and required additional articulation. Lest you say that since it reignited, it returns to its original status and is prohibited, therefore it taught us that this is not the case.

אמר רבה בר בר חנה אמר רבי יוחנן: קטמה והובערה, משהין עליה חמין שהוחמו כל צורכן ותבשיל שבישל כל צורכו, ואפילו גחלים של רותם. שמע מינה מצטמק ויפה לו מותר! שאני הכא דקטמה. אי הכי מאי למימרא?! הובערה אצטריכא ליה. היינו הך! גחלים של רותם אצטריכא ליה.

Rabba bar bar Ḥana said that R' Yoḥanan said: With regard to a stove that he swept out or covered with ashes before Shabbat and subsequently reignited on Shabbat, one may leave hot water that was already completely heated and cooked food that was already completely cooked upon it, even if the coals were from the wood of a broom tree, which are very hot and long-burning. If so, conclude from this that even if food shrivels and improves while on the stove, it is permitted. The Talmud rejects this: Here, in this case, it is different because he covered it with ashes. Therefore, it is permitted to leave it on the stove. The Talmud asks: If so, what was the purpose of saying this halakha? The Talmud answers: Mention of the case where he covered it with ashes and it reignited on Shabbat was necessary. The Talmud challenges that explanation: This case is identical to the previous one. Why did R' Yoḥanan find it necessary to repeat what was already said? The Talmud responds that there is a novel element in his statement. It was necessary to teach the case of coals from the wood of a broom tree. Even in a case of especially hot coals it is permitted.