Shabbat 39A

כל שבא בחמין מלפני השבת ⁠— שורין אותו בחמין בשבת, וכל שלא בא בחמין מלפני השבת ⁠— מדיחין אותו בחמין בשבת. חוץ מן המליח ישן וקולייס האיספנין, שהדחתן זו היא גמר מלאכתן. שמע מינה.

Any salted food item that was already placed in hot water, i.e., cooked, before Shabbat, one may soak it in hot water even on Shabbat. And anything that was not placed in hot water before Shabbat, one may rinse it in hot water on Shabbat, but may not soak it, with the exception of old salted fish or the colias of the Spaniards [kolyas ha’ispanin] fish, for which rinsing with hot water itself is completion of the prohibited labor of cooking. Once it is rinsed in hot water, it does not require any additional cooking. The same is true with regard to an egg that was slightly cooked. Since it thereby becomes edible, one who brought it to that state has violated the prohibition of cooking. The Talmud sums up: Indeed, conclude from it that this is its meaning.

ולא יפקיענה בסודרין: והא דתנן נותנין תבשיל לתוך הבור בשביל שיהא שמור, ואת המים היפים ברעים בשביל שיצננו, ואת הצונן בחמה בשביל שיחמו ⁠— לימא רבי יוסי היא ולא רבנן?

We also learned in the Mishnah according to the first tanna: And one may not wrap an egg in cloths that were heated by the sun in order to heat up the egg, and R' Yosei permits doing so. And with regard to that which we learned in a Mishnah that one may place cooked food into a pit on Shabbat to protect it from the heat; and one may place good, potable water into bad, non-potable water so that it will cool; and one may put cold water out in the sun to heat it, the Talmud asks: Let us say that this Mishnah is in accordance with the opinion of R' Yosei in our Mishnah and not the opinion of the Rabbis as represented by the first tanna in the Mishnah. The Rabbis prohibited heating food with the heat of the sun.