Is it correct to say, "Beware the evil of the forbearing one when he is angered," and is there any prohibition in it?

There is nothing in this common proverb that necessitates prohibiting it. What is meant by it is that a forbearing person may lose his temper and become excessively angry if provoked, and inflict upon his provocateur an evil he did not anticipate. It is thus a guidance against being deluded by the forbearance of the forbearing and provoking him with what might make him act contrary to his nature and custom.