No punishment is imposed unless there is prior warning
A legal principle stating that punishment cannot be imposed before a prior warning has been given about the expected penalty for violating the law.
("Is it not logical that there should be no punishment unless there was a warning? But as for the warning for the affliction of the day itself, from where do we derive it?... By analogy: it is stated there is punishment for affliction, and it is stated there is punishment for work. Just as for work—no punishment unless there was a warning, so for affliction—no punishment unless there was a warning."—Yoma 81a, and also: "So Rashi explained, namely, as is said elsewhere: 'No punishment is imposed unless there is warning.'"—Tosafot Yom Tov, Keritot, Chapter 2)
Translation
Expression (Hebrew)
אֵין עֳנָשִׁין אֶלָּא אִם כֵּן מַזְהִירִין
Definition (Hebrew)
כְּלָל מִשְׁפָּטִי לְפִיו לֹא נִתָּן לְהַטִּיל עֹנֶשׁ לִפְנֵי שֶׁנָּתְנוּ אַזְהָרָה קֹדֶם לָכֵן עַל הָעֹנֶשׁ הַצָּפוּי לְמִי שֶׁיַּעֲבֹר עַל הַחֹק. כְּלָל מִשְׁפָּטִי הַמֻּכָּר בְּלָטִינִית: "Nullum crimen sine lege"
("אֵינוֹ דִּין שֶׁלֹּא יְהֵא מֻזְהָר עָלָיו? אֲבָל אַזְהָרָה לָעִנּוּי שֶׁל יוֹם עַצְמוֹ לֹא לָמַדְנוּ מִנַּיִן...מִמֶּנּוּ גְּזֵרָה שָׁוָה: נֶאֱמַר עֹנֶשׁ בְּעִנּוּי, וְנֹאמַר עֹנֶשׁ בַּמְּלָאכָה. מָה מְלָאכָה-לֹא עָנַשׁ אֶלָּא אִם כֵּן הִזְהִיר, אַף עִנּוּי-לֹא עָנַשׁ אֶלָּא אִם כֵּן הִזְהִיר"- יוֹמָא פא ע"א, וְכֵן:"וְכֵן פירש"י וּכְלוֹמַר כדאמרינן בְּעָלְמָא אֵין עוֹנְשִׁין אֶלָּא אִם כֵּן מַזְהִירִין"- תּוֹסָפוֹת יוֹם טוֹב מַסֶּכֶת כְּרִיתוּת פֶּרֶק ב)