Enchiridion · §9

νόσος σώματός ἐστιν ἐμπόδιον, προαιρέσεως δὲ οὔ, ἐὰν μὴ αὐτὴ θελήσῃ. χωλότης σκέλους ἐστὶν ἐμπόδιον, προαιρέσεως δὲ οὔ. καὶ τοῦτο ἐφ᾽ ἑκάστου τῶν ἐμπιπτόντων ἐπίλεγε· εὑρήσεις γὰρ αὐτὸ ἄλλου τινὸς ἐμπόδιον, σεαυτοῦ δὲ οὔ.

nosos sōmatos estin empodion, proaireseōs de ou, ean mē autē thelēsē. chōlotēs skelous estin empodion, proaireseōs de ou. kai touto eph' hekastou tōn empiptontōn epilege; heurēseis gar auto allou tinos empodion, seautou de ou.

Illness is an impediment of the body, but not of the will — unless the will chooses to be impeded. Lameness is an impediment of the leg, but not of the will. Say this to yourself with regard to everything that happens to you: you will find that it is an obstacle to something else, but not to you yourself.

— Enchiridion 9

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