אורח חיים תקע״אOrach Chayim 571

Se’if 1א׳

הַיּוֹשֵׁב בְּתַעֲנִית, אִם יָכוֹל לִסְבֹּל הַתַּעֲנִית נִקְרָא קָדוֹשׁ; וְאִם לָאו, כְּגוֹן שֶׁאֵינוֹ בָּרִיא וְחָזָק, נִקְרָא חוֹטֵא.

One who sits in a fast: if he is able to bear the fast, he is called holy; and if not — for instance, where he is not healthy and strong — he is called a sinner.

Se’if 2ב׳

תַּלְמִיד חָכָם אֵינוֹ רַשַּׁאי לִישֵׁב בְּתַעֲנִית, מִפְּנֵי שֶׁמְּמַעֵט בִּמְלֶאכֶת שָׁמַיִם, אֶלָּא אִם כֵּן כְּשֶׁהַצִּבּוּר מִתְעַנִּים שֶׁלֹּא יִפְרֹשׁ עַצְמוֹ מֵהֶם; וּמְלַמְּדֵי תִּינוֹקוֹת דִּינָם כְּתַלְמִידֵי חֲכָמִים.

A Torah scholar is not permitted to sit in a fast, because he diminishes the work of Heaven, unless the congregation is fasting, so that he should not separate himself from them; and teachers of young children — their law is as that of Torah scholars.

Se’if 3ג׳

עִיר שֶׁהִקִּיפוּהָ עַכּוּ"ם, וְכֵן יָחִיד הַנִּרְדָּף מִפְּנֵי עַכּוּ"ם אוֹ מִפְּנֵי רוּחַ רָעָה, אֵינָם רַשָּׁאִים לְהִתְעַנּוֹת, שֶׁלֹּא לְשַׁבֵּר כֹּחָם; אֶלָּא יְקַבְּלוּ עֲלֵיהֶם לְהִתְעַנּוֹת כָּךְ וְכָךְ תַּעֲנִיּוֹת לִכְשֶׁיִּנָּצְלוּ; וְכֵן אִם גָּזְרוּ גְּזֵרָה וְגָזְרוּ שֶׁלֹּא לְהִתְעַנּוֹת עָלָיו, יְקַבְּלוּ עֲלֵיהֶם לְהִתְעַנּוֹת כָּךְ וְכָךְ תַּעֲנִיּוֹת כְּשֶׁיַּעֲבֹר הַגְּזֵרָה.

A city that gentiles have surrounded, and likewise an individual who is pursued by gentiles or by an evil spirit — they are not permitted to fast, so as not to break their strength; rather they should accept upon themselves to fast so-and-so many fasts when they are rescued. And likewise, if a decree was decreed and it was decreed that they should not fast over it, they should accept upon themselves to fast so-and-so many fasts when the decree passes.