John Cassian, The Institutes VIII.21

Whether we ought to admit the addition of “without a cause,” in that which is written in the Gospel, “whosoever is angry with his brother,” etc.

But you should know that in this, which is found in many copies, “Whosoever is angry with his brother without a cause, is in danger of the judgment,” the words “without a cause” are superfluous, and were added by those who did not think that anger for just causes was to be banished: since certainly nobody, however unreasonably he is disturbed, would say that he was angry without a cause. Wherefore it appears to have been added by those who did not understand the drift of Scripture, which intended altogether to banish the incentive to anger, and to reserve no occasion whatever for indignation; lest while we were commanded to be angry with a cause, an opportunity for being angry without a cause might occur to us. For the end and aim of patience consists, not in being angry with a good reason, but in not being angry at all. Although I know that by some this very expression, “without a cause,” is taken to mean that he is angry without a cause who when he is angered is not allowed to seek for vengeance. But it is better so to take it as we find it written in many modern copies and all the ancient ones.