Yosef Karo, Shulchan Aruch — Choshen Mishpat 9
“ Paragraph 5- A judge who accepts payment in order to judge has his rulings voided except for cases where we specifically know he did not accept payment. If he simply accepts payment for being idle, that is permissible so long as it is obvious to all, such as a case where he has known work to perform at that time and he tells the parties to give him the cost of that work that he is missing. ”
