Nemo judex in causa sua (or nemo judex in sua causa) is a Latin phrase that means, literally, “no-one should be a judge in his own cause.” It is a principle of natural justice that no person can judge a case in which they have an interest.
Gemara Bava Basra 43a (from Tosefta):
בני עיר שנגנב ספר תורה שלהן אין דנין בדייני אותה העיר ואין מביאין ראיה מאנשי אותה העיר
The rule is very strictly applied to any appearance of a possible bias, even if there is actually none: “Justice must not only be done, but must be seen to be done”.
והייתם נקיים מה’ ומישראל