אָר֕וּר מַכֵּ֥ה רֵעֵ֖הוּ בַּסָּ֑תֶר וְאָמַ֥ר כָּל־הָעָ֖ם אָמֵֽן׃ (ס)
Cursed be he that smiteth his neighbour in secret. And all the people shall say: Amen.
Read kabbalistic (mystical) interpretation of Deuteronomy 27:24: and classical mystical sources.