סימן פו סעיף ב

הַמַּיִם שֶׁהֵם מִמְּקוֹרָם נוֹבְעִים חַמִּים, כְּמוֹ חַמֵּי טְבֶרְיָה וְכַדּוֹמֶה, אִם הֵם בַּקַּרְקַע וְאֵין הַמָּקוֹם מְקוֹרֶה, מֻתָּר לִרְחוֹץ בָּהֶם אֲפִלּוּ כָּל גּוּפוֹ. אֲבָל אִם הֵם בִּכְלִי אוֹ שֶׁהַמָּקוֹם מְקוֹרֶה אָסוּר. וַאֲפִלּוּ לִכָּנֵס לְבֵית הַמֶּרְחָץ רַק לְהַזִּיעַ, אָסוּר. וְיֵשׁ אוֹמְרִים, דְּכָל שֶׁהֵם בַּקַּרְקַע, אֲפִלּוּ הַמָּקוֹם מְקֹרֶה, מֻתָּר לִרְחֹץ בָּהֶם.

Water that flows hot from its source such as the hot springs of Tiberias and the like, if [the waters] are in the ground and there is no roof over them, you may wash even your entire body⁴Only water heated by fire is prohibited. [Maseches Shabbos 40] in them. But, if [the water] is in a vessel⁵In such a case, the hot water may be confused with water heated by fire. or the place is covered with a roof, it is forbidden.⁶The roof increases the heat of the room, making it akin to a “shvitz” (sauna room) which may not be used on Shabbos. The Mishnah Berurah, however, (citing Shulchan Aruch 326: 22) rules that you may use heat from hot springs for sweating since even bathing is permitted there. Even to enter a bathhouse solely for the purpose of sweating, is forbidden.⁷Because some who are unscrupulous may go to bathe and claim that they are going only to sweat.
The words of the author imply that merely passing through a bathhouse is permitted which is the opinion of Shulchan Aruch (326: 12). However, Rema (from Rashi and Tur) rules that even just passing through without intention to sweat is prohibited. There are those who say that as long as [the water] is in the ground even if the place has a roof, you may bathe in it.

תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)