וְלֹ֤א יִשָּׁפֵךְ֙ דָּ֣ם נָקִ֔י בְּקֶ֣רֶב אַרְצְךָ֔ אֲשֶׁר֙ יְהוָ֣ה אֱלֹהֶ֔יךָ נֹתֵ֥ן לְךָ֖ נַחֲלָ֑ה וְהָיָ֥ה עָלֶ֖יךָ דָּמִֽים׃ (ס)
that innocent blood be not shed in the midst of thy land, which the LORD thy God giveth thee for an inheritance, and so blood be upon thee.
Sifrei Devarim
(Devarim 19:10) "And innocent blood shall not be shed in the midst of your land that the L-rd your G-d gives to you as an inheritance": Beth-din is hereby being exhorted to this end.