Moses relates how he appointed ethical judges to assist him in judging Israel, and along with other instructions, he commanded them not to give preference in judgment. The Sages understand this command on two levels: that judges should not give preference to one of the parties for immaterial reasons, and that the appointment of the judges themselves should be for substantive reasons, without favoritism.

When two litigants come before you for judgment, one poor and one wealthy, do not say: How can I find in favor of the poor man and find the wealthy man liable [as that would insult a prominent person], or: How can I find in favor of the wealthy man and find the poor man liable [as that would cause hardship to a needy person]? If I find the poor man liable, the poor man will be my enemy; and if I find in favor of the poor man, the wealthy man will be my enemy. Do not say: How can I take this man’s property and give it to that man? The Torah says: “You shall not give preference in judgment; small and great alike you shall hear” (Deuteronomy 1:17).

“You shall not give preference in judgment.” This is directed at the official tasked with appointing judges. Should he say: This person is attractive, I will appoint him as a judge; or: This man is powerful, I will appoint him as a judge; or: This man is my relative, I will appoint him as a judge; or: This man lent me money, I will appoint him as a judge? If he were to appoint a judge for one of those reasons, the judge would thereby find in favor of the guilty and find the innocent liable; not because the judge is wicked, but because he is ignorant. The Torah ascribes guilt to that official as though it was he who gave preference in judgment.

(Avot deRabbi Natan, version A, chap. 10; Sifrei Devarim, Devarim 17)