Enchiridion · §42

ὅταν τίς σε κακῶς ποιῇ ἢ κακῶς λέγῃ, μέμνησο ὅτι κακῶς ποιεῖν ἢ λέγειν αὐτῷ καθήκειν δοκεῖ. ἀδύνατον οὖν ᾧ φαίνεται ἐκεῖνο τοῦτο μὴ ἀκολουθεῖν. εἰ οὖν τοῦτο ψεῦδος, ὁ βλαπτόμενος ὑπ᾽ αὐτοῦ ἐστιν. ὡς οὖν ταῦτα λογίζου· ὁ μὲν ψεύδεται, ὁ δὲ κατελήφθη.

hotan tis se kakōs poiē ē kakōs legē, memnēso hoti kakōs poiein ē legein autō kathēkein dokei. adunaton oun hō phainetai ekeino touto mē akolouthein. ei oun touto pseudos, ho blaptomenos hup' autou estin.

When someone treats you badly or speaks badly of you, remember that they are acting or speaking as they do because it seems right to them. It is impossible for a person not to act according to how things appear to them. If what they believe is false, then the one who is harmed is the person deceived by that false belief — not you.

— Enchiridion 42

Source: Greek (Schenkl/Oldfather critical edition tradition) + Long 1877 English translation