הלכה על מכות 45:21

To not hear the claim of a litigant when it is not in front of his fellow litigant: That the judge not hear the claim of one, not in front of his adversary, as it is stated (Exodus 23:1), "You shall not raise a false report." And the reason is because people will speak idle words when not in front of their adversary. And the judge is commanded about this so that he not bring the untruths of one of them into his soul. And so does Mekhilta d'Rabbi Yishmael 23:1 come [to tell us] that this warning of "You shall not raise, etc." is said about this. And they also said there that it is also a warning to the litigant, for him too, not to make his claims to the judge not in front of his adversary, and even if the judge wants to hear it. And about this, it is also said (Exodus 23:7), "From a false matter, distance yourself." And they, may their memory be blessed, also said (Makkot 23a) that this negative commandment includes telling evil speech, and accepting it, and giving false testimony.