Enchiridion 42

When someone treats you ill or speaks ill of you, remember that he acts or speaks thus because he thinks it is incumbent upon him. That being the case, it is impossible for him to follow what appears good to you, but what appears good to himself; whence it follows, that, if he gets a wrong view of things, the man that suffers is the man that has been deceived. For if a person thinks a true composite judgement to be false, the composite judgement does not suffer, but the person who has been deceived. If, therefore, you start from this point of view, you will be gentle with the man who reviles you. For you should say on each occasion, "He thought that way about it."

Epictetus, Enchiridion 42, translated by W. A. Oldfather

When any person treats you ill or speaks ill of you, remember that he does this or says this because he thinks that it is his duty. It is not possible then for him to follow that which seems right to you, but that which seems right to himself. Accordingly if he is wrong in his opinion, he is the person who is hurt, for he is the person who has been deceived; for if a man shall suppose the true conjunction to be false, it is not the conjunction which is hindered, but the man who has been deceived about it. If you proceed, then, from these opinions, you will be mild in temper to him who reviles you: for say on each occasion, It seemed so to him.

Epictetus, Enchiridion 42, translated by George Long

When a man speaks evil or does evil to you, remember that he does or says it because he thinks it is fitting for him. It is not possible for him to follow what seems good to you, but only what seems good to him, so that, if his opinion is wrong, he suffers, in that he is the victim of deception. In the same way, if a composite judgement which is true is thought to be false, it is not the judgement that suffers, but the man who is deluded about it. If you act on this principle you will be gentle to him who reviles you, saying to yourself on each occasion, 'He thought it right.'

Epictetus, Enchiridion 42, translated by P. E. Matheson