Shabbat 39B

ממעשה שעשו אנשי טבריא ואסרי להו רבנן בטלה הטמנה בדבר המוסיף הבל, ואפילו מבעוד יום. אמר עולא: הלכה כאנשי טבריא. אמר ליה רב נחמן: כבר תברינהו אנשי טבריא לסילונייהו.

From this action performed by the people of Tiberias and the fact that the rabbis prohibited them from using the water, the conclusion is that the practice of insulating a pot in something that increases the heat over the course of Shabbat was abolished on Shabbat. And not only is it prohibited to do so on Shabbat itself, but it is also prohibited while it is still day before Shabbat. Running pipes of cold water through hot water is similar to insulating water in something that adds heat. Ulla said: The halakha is in accordance with the people of Tiberias. Rav Naḥman said to him: The people of Tiberias have already broken their pipes. Even they reconsidered their position.

מעשה שעשו אנשי טבריא: מאי רחיצה? אילימא רחיצת כל גופו ⁠— אלא חמין שהוחמו בשבת הוא דאסורין, הא חמין שהוחמו מערב שבת מותרין? והתניא: חמין שהוחמו מערב שבת, למחר רוחץ בהן פניו ידיו ורגליו, אבל לא כל גופו! אלא פניו ידיו ורגליו.

We learned in the Mishnah with regard to the incident, which related what the people of Tiberias did, that the legal status of water that was heated in the Tiberias hot springs is like that of water heated on Shabbat, and it is prohibited for use in bathing. The Talmud clarifies this matter: What type of bathing is this? If you say that it is referring to bathing one’s entire body, that is difficult. That would indicate that only water heated on Shabbat is prohibited for use in bathing one’s entire body; however, bathing one’s entire body in hot water heated before Shabbat is permitted. That cannot be. Wasn’t it taught in a baraita: With regard to hot water that was heated on Friday, one may use it the next day to wash his face, his hands, and his feet incrementally; however, not to wash his entire body? Rather, it must be that the bathing prohibited in the Mishnah with water heated on Shabbat is, in fact, washing his face, his hands, and his feet.