St. Isaac the Syrian, Homily 51

Speak circumspectly before him who is conceited and diseased with envy. For while you are speaking, he is interpreting your words as he pleases, and gathers fuel from what is good in you to make others stumble, and in his mind changes your words to suit the substance of his illnes as soon as he begins to speak evil of his brother in your presence, show a gloomy countenance. When you do this, you will be found prudent before God and before him. - St. Isaac the Syrian, homily 51