הלכה על סנהדרין 12:12

The laws of the commandment are that which they said (Sanhedrin 6b), "Two [litigants] that come before you in judgment - one is gentle and one is difficult - before you hear their statements, or even after you hear their statements but you do not yet know where the judgment is leaning, it is permitted for you to say to them, 'I am not available to you,' lest he will be found liable, and it will turn out that he will pursue after the judge to kill him. But once you hear their statements and you know where the judgment is leaning, you may not say to them, 'I am not available to you,' as it is stated, 'Do not fear any man.'" And so [too] (Sanhedrin 6b), "A student who is sitting before his teacher and he sees innocence for a poor person and guilt for a wealthy person is not allowed to be silent, as it is stated, 'Do not fear any man.'"