Musar על ברכות 58:19

And you often see people who when they are angry and persist in their wrath, are not conscious of what they are doing and do many things in their anger which they would not do if they were free from anger, for anger draws out the intelligence of a person from within him until his angry deeds multiply and he is plunged into strife and quarrel. Therefore, it is impossible that the wrathful person should be saved from great sins. And so did Elijah say to Rabbi Judah : "Fall not into a passion and you will not sin" (Berakoth 29b). And the Sages said : "By three things is a man known" (Erubin 65b) and one of them is his anger, for when a man is angry his true nature can be recognized. If his wrath is stronger than his wisdom and he does things in the moment of his anger without regard to his wisdom, then you can see the character of his wrath. But if his wisdom is stronger than his anger and he de does not say or do anything when angry that he would not say or do when he is free from anger, then you can see the extent of his wisdom. And the Wise Man said : "Three, the Holy One Blessed be He loves and one of them is he who does not anger" (Pesahim 113b). And our Sages said : "Nor is an impatient person fitted to teach" (Aboth 2:5), for because of his great anger, the pupils fear him too much to ask the things of which they are in doubt lest he be wrathful with them. And even when his pupils do ask the impatient teacher questions he has neither the mind nor the patience to explain to his pupils all that is needed to make the matter clear. Then, too, he will answer questions in anger and thus the pupils will not understand the matter clearly. As for the pupils, it is their duty even when their teacher is angry with them, to ask their questions and listen carefully and not be hurt by the anger of their teacher nor quarrel with him. Concerning such pupils our Sages taught : "So the forcing of wrath bringeth forth strife" (Proverbs 30:33). Any pupil whose teacher is angry with him repeatedly but bears the teacher's wrath in silence will merit to discern between civil and criminal law. And a master said : "There is nothing more difficult than civil and criminal Law" (cf. Berakoth 63b).