Musar על שבת 254:12

If you see a man who speaks a word or does a deed which can be interpreted either favorably or unfavorably, then if he is a man who reveres God you are obliged to give him the benefit of the doubt, even if the unfavorable interpretation appears more likely. And if he is an ordinary person who guards himself from sin, but occasionally stumbles, it is still your duty to put doubt aside and decide his favor. And our Sages, of blessed memory, said: "He who judges his neighbor in the scale of merit is himself judged favorably" (Shabbath 127b). And this is a positive commandment in the Torah, as it is said, "But in righteousness shalt thou judge thy neighbor" (Lev. 19:15). And if the matter inclines to the unfavorable interpretation, let it be with you as though there were a doubt and do not judge the man unfavorably. But if the man's deeds for the most part are evil, and you know that he is not one who reveres God in his heart, then you should put the unfavorable interpretation on his deeds and words.