Enchiridion · §20

μέμνησο ὅτι οὐχ ὁ λοιδορῶν ἢ ὁ τύπτων ὑβρίζει, ἀλλὰ τὸ δόγμα τὸ περὶ τούτων ὅτι ὑβρίζει. ὅταν οὖν ἐρεθίσῃ σέ τις, ἴσθι ὅτι ἡ σὴ ὑπόληψίς σε ἠρέθισεν. τοιγαροῦν ἐν πρώτοις πειρῶ μὴ ἐξαρπασθῆναι ὑπὸ τῆς φαντασίας· ἅπαξ γὰρ χρόνου λαβόμενος εὐκολώτερον κρατήσεις σεαυτοῦ.

memnēso hoti ouch ho loidorōn ē ho tuptōn hubrizei, alla to dogma to peri toutōn hoti hubrizei. hotan oun erethisē se tis, isthi hoti hē sē hupolēpsis se ērethisen. toigaroun en prōtois peirō mē exarpasthēnai hupo tēs phantasias.

Remember that it is not the person who reviles or strikes you that insults you, but your own judgment that these things are insulting. Therefore, when someone provokes you, know that it is your own opinion that has provoked you. Above all, try not to be swept away by the initial impression — for if you can gain time, you will more easily master yourself.

— Enchiridion 20

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