אורח חיים שכ״וOrach Chayim 326

Se’if 1א׳

אָסוּר לִרְחֹץ כָּל גּוּפוֹ, אֲפִלּוּ כָּל אֵבֶר וְאֵבֶר לְבַד, אֲפִלּוּ בְּמַיִם שֶׁהֻחַמּוּ מֵעֶרֶב שַׁבָּת, בֵּין אִם הֵם בִּכְלִי בֵּין אִם הֵם בַּקַּרְקַע; וַאֲפִלּוּ לִשְׁפֹּךְ הַמַּיִם עַל גּוּפוֹ וּלְהִשְׁתַּטֵּף, אָסוּר; אֲבָל מֻתָּר לִרְחֹץ בָּהֶם פָּנָיו יָדָיו וְרַגְלָיו. הַגָּה: אוֹ שְׁאָר אֵבָרִים, כָּל שֶׁאֵינוֹ רוֹחֵץ כָּל גּוּפוֹ (בֵּית יוֹסֵף בְּשֵׁם הָרֹא"שׁ פֶּרֶק תִּינֹקֶת). וְהָנֵי מִלֵּי בְּחַמֵּי הָאוּר, אֲבָל בְּחַמֵּי טְבֶרְיָא מֻתָּר לִרְחֹץ אֲפִלּוּ כָּל גּוּפוֹ יַחַד, וְאֵין צָרִיךְ לוֹמַר בְּצוֹנְנִין. וְהָא דְּשָׁרֵי בְּחַמֵּי טְבֶרְיָא, דַּוְקָא בַּקַּרְקַע, אֲבָל בִּכְלִי, לֹא, דְּאָתֵי לְאִחְלוּפֵי בְּחַמֵּי הָאוּר.

The Laws of Bathing on Shabbat. Contains 13 Seifim: One is forbidden to wash his entire body, even when washing one limb at a time, even when using water that was heated before Shabbos, whether the water is in a vessel or in the ground. Even pouring water over oneself and washing is forbidden. Nevertheless one may wash his face hands and feet Rema: or other limbs as long as he does not wash his entire body. [Beis Yosef in the name of the Rosh] The above pertains to water heated by fire but one may wash his entire body with Tiberius hot spring water and needless to say that he may wash in cold water. One may only wash in Tiberius hot spring water that is in the ground but not in a vessel so as not to confuse it with regular hot water.

Se’if 2ב׳

יֵשׁ אוֹמְרִים דְּהָא דְּשָׁרֵי בְּחַמֵּי טְבֶרְיָא דַּוְקָא כְּשֶׁאֵין הַמָּקוֹם מְקֹרֶה, אֲבָל אִם הַמָּקוֹם מְקֹרֶה, אָסוּר מִשּׁוּם דְּאָתֵי לִידֵי זֵעָה וְאָסוּר. וְיֵשׁ אוֹמְרִים דְּמֻתָּר לְהַזִּיעַ בְּחַמֵּי טְבֶרְיָא.

There are those who say that this is permissible specifically in the hot springs of Tiberius unless the place [where the water is located] has no roof. But if the place has a roof it is forbidden because [the person] will sweat, and it is forbidden to [cause oneself] to sweat on Shabbos. And there are those who say that it is permitted to sweat in the hot springs of Tiberius.

Se’if 3ג׳

אַמַּת הַמַּיִם שֶׁהִיא חַמָּה, אָסוּר לְהַמְשִׁיךְ לְתוֹכָהּ אֲפִלּוּ מֵעֶרֶב שַׁבָּת סִילוֹן (פֵּרוּשׁ צִנּוֹר מַרְזֵב וְסִילוֹן דָּבָר אֶחָד הֵם) שֶׁל צוֹנֵן, וּפִי הַסִילוֹן יוֹצֵא חוּץ לָאַמָּה וּמֵימָיו נִשְׁפָּכִים לְעוּקָא (פֵּרוּשׁ גּוּמָא) שֶׁבַּקַּרְקַע; וְאִם הִמְשִׁיכוּ, אֲסוּרִים. אֲפִלּוּ הַמַּיִם שֶׁנִּכְנְסוּ לָהּ מֵעֶרֶב שַׁבָּת בִּרְחִיצָה וּבִשְׁתִיָּה, כְּאִלּוּ הֻחַמּוּ בְּשַׁבָּת; וְאִם הֵבִיא סִילוֹן שֶׁל מַיִם מֵעֶרֶב יוֹם טוֹב בְּיוֹם טוֹב, כְּחַמִּין שֶׁהֻחַמּוּ בְּיוֹם טוֹב וַאֲסוּרִין בִּרְחִיצָה וּמֻתָּרִין בִּשְׁתִיָּה.

When there is a water channel in which water from a natural hot spring flows it is forbidden to run a duct of cold water through it, even from Friday, i.e., the source of the duct’s water is located outside the water channel and its water flows into a [nearby] cavity in the ground, [creating a pool in which to bathe]. (The intent is to warm the water in the duct as it passes through the hot spring’s water). And if one does run (such a duct) it is forbidden, even to use the water that enters it from Friday. If [water] was directed to flow through the duct on the eve of a festival, it is governed by the laws that apply to water that was heated on a festival, i.e., it is forbidden to bathe one’s entire body or the majority of it in [such water]; it is forbidden to rinse one's body in it but it is permitted to drink.