Shabbos 3:4-5

Shabbos 3:4

The men of Tiberias once directed a pipe of cold water into a channel of hot water (in order to heat it). The Sages told them that any water heated on Shabbos may not be used for washing or drinking, and any water heated on yom tov may not be used for washing but may be used for drinking. If the coals of a samovar are raked before Shabbos, one may drink from it on Shabbos. Regarding an antiki (another type of heated vessel), one may not drink from it on Shabbos even if the coals had been raked beforehand (because it continues to heat on Shabbos).

Shabbos 3:5

If an urn was removed from the fire, one may not pour from it into cold water in order to heat it, but one may add cold water to it in order to make it lukewarm; one may likewise pour the hot water into a cup and then add cold water to it. If one removed a boiling pot of food from the stove, he may not add spices to it (because this will cook them) but he may add spices to a dished-out plate of food or into a serving dish of food. Rabbi Yehuda permits one to add spices directly to the hot pot except for things containing vinegar or brine (which would facilitate cooking).