Positive commandment · Mitzvat Aseh #177 of 613
To treat litigants equally when they appear [in court] to be judged
להשוות בין בעלי דינין בשעה שעומדין בדין
The verse · Vayikra 19:15
לֹא־תַעֲשׂוּ עָוֶל בַּמִּשְׁפָּט לֹא־תִשָּׂא פְנֵי־דָל וְלֹא תֶהְדַּר פְּנֵי גָדוֹל בְּצֶדֶק תִּשְׁפֹּט עֲמִיתֶךָ
“You shall do no unrighteousness in judgment: thou shalt not respect the person of the poor, nor honour the person of the mighty: but in righteousness shalt thou judge thy neighbour.”
The Rambam’s words
To treat litigants equally when they appear [in court] to be judged, as [Leviticus 19:15] states: "Judge your fellow man with righteousness."
להשוות בין בעלי דינין בשעה שעומדין בדין, שנאמר: "בצדק, תשפוט עמיתך" (ויקרא יט,טו).
Why · Sefer HaChinuch #235
Judge with righteousness
“The root of the commandment is revealed to all — as with equitability in judgment, the world is civilized. But if a judge honors one of the litigants over the other, the [latter] disputant will fear to say all of his claims in front of him.”
Who & when
“And this commandment is practiced in every place and at all times by males — as they are obligated to judge, and not females.”
Where in the Mishneh Torah
Hilchot The Sanhedrin and the Penalties within Their Jurisdiction הלכות סנהדרין והעונשין המסורין להם