Negative commandment · Lo Ta’aseh #283 of 613
For a person who argued in favor of acquittal in a capital case not to argue for a conviction
שלא ילמד חובה מי שלימד זכות תחילה בדיני נפשות
The verse · Shemot 23:2
לֹא־תִהְיֶה אַחֲרֵי־רַבִּים לְרָעֹת וְלֹא־תַעֲנֶה עַל־רִב לִנְטֹת אַחֲרֵי רַבִּים לְהַטֹּת
“Thou shalt not follow a multitude to do evil; neither shalt thou speak in a cause to incline after a multitude to pervert justice:”
The Rambam’s words
For a person who argued in favor of acquittal in a capital case not to argue for a conviction, as [Exodus, ibid.] states: "Do not speak up in a trial to influence...."
שלא ילמד חובה מי שלימד זכות תחילה בדיני נפשות, שנאמר "לא תענה על ריב, לנטות" (שמות כג, ב).
Why · Sefer HaChinuch #77
That the one who advocated innocence at the beginning of capital cases not advocate guilt
“From the roots of this commandment is like that which we said at the beginning — that one of the judges not go after his colleagues, but rather he should understand the things on his own.”
Who & when
“And this commandment is practiced only in the Land of Israel, as we only judge capital cases there.”
Where in the Mishneh Torah
Hilchot The Sanhedrin and the Penalties within Their Jurisdiction הלכות סנהדרין והעונשין המסורין להם