Negative commandment · Lo Ta’aseh #281 of 613
Not to listen to one litigant in the absence of the other
שלא לשמוע מאחד מבעלי דינין ואין חברו עימו
The verse · Shemot 23:1
לֹא תִשָּׂא שֵׁמַע שָׁוְא אַל־תָּשֶׁת יָדְךָ עִם־רָשָׁע לִהְיֹת עֵד חָמָס
“Thou shalt not raise a false report: put not thy hand with the wicked to be an unrighteous witness.”
The Rambam’s words
Not to listen to one litigant in the absence of the other, as [Exodus 23:1] states: "Do not hear a false report."
שלא לשמוע מאחד מבעלי דינין ואין חברו עימו, שנאמר "לֹא תִשָּׂא שֵׁמַע שָׁוְא" (שמות כג, א).
Why · Sefer HaChinuch #74
To not hear the claim of a litigant when it is not in front of his fellow litigant
“The root of this commandment is well-known, as falsehood is abominable and vile in the eyes of all. There is nothing more disgusting than it, and malediction and curse are in the house of its lovers.”
Who & when
“And [it] is practiced in every place and at all times by males, but not by females, since they do not judge.”
Where in the Mishneh Torah
Hilchot The Sanhedrin and the Penalties within Their Jurisdiction הלכות סנהדרין והעונשין המסורין להם