Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 177
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 177, with English translation by Maimonides (Rambam).
להשוות בין בעלי דינין בשעה שעומדין בדין, שנאמר: "בצדק, תשפוט עמיתך" (ויקרא יט,טו).
To treat litigants equally when they appear [in court] to be judged, as [Leviticus 19:15] states: "Judge your fellow man with righteousness."