Enchiridion · §10

ἐφ᾽ ἑκάστου τῶν προσπιπτόντων μέμνησο ἐπιστρέφων ἐπὶ σαυτὸν ζητεῖν, τίνα δύναμιν ἔχεις πρὸς τὴν χρῆσιν αὐτοῦ. ἐὰν καλὸν ἴδῃς ἢ καλήν, εὑρήσεις τὴν δύναμιν πρὸς αὐτὸ τὴν ἐγκράτειαν· ἐὰν πόνος προσφέρηται, εὑρήσεις τὴν καρτερίαν· ἐὰν λοιδορία, εὑρήσεις τὴν ἀνεξικακίαν. καὶ οὕτως ἐθιζόμενός σε οὐ συναρπάσουσιν αἱ φαντασίαι.

eph' hekastou tōn prospiptontōn memnēso epistrephōn epi sauton zētein, tina dunamin echeis pros tēn chrēsin autou. ean kalon idēs ē kalēn, heurēseis tēn dunamin pros auto tēn enkrateian; ean ponos prospherētai, heurēseis tēn karterian; ean loidoria, heurēseis tēn aneksikakian.

With every challenge that confronts you, turn inward and ask yourself: what faculty do I possess to meet this? If you see someone beautiful, self-restraint is the faculty you need. If pain approaches, endurance is what you have. If someone insults you, patience is your resource. Practiced in this way, the raw force of appearances will no longer carry you away.

— Enchiridion 10

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