Enchiridion · §13

εἰ θέλεις προκόπτειν, ὕπομεινον ἕνεκα τῶν ἐκτὸς ἀνόητος καὶ μωρὸς δοκῶν· μηδὲν βούλου δοκεῖν ἐπίστασθαι· κἂν δόξῃς τισὶ εἶναί τις, ἀπίστει σαυτῷ. ἴσθι γὰρ ὅτι οὐ ῥᾴδιον τήν τε προαίρεσιν τὴν σεαυτοῦ κατὰ φύσιν ἔχουσαν φυλάξαι καὶ τὰ ἐκτός· ἀλλ᾽ ἀνάγκη τοῦ ἑτέρου ἐπιμελούμενον τοῦ ἑτέρου ἀμελεῖν.

ei theleis prokoptein, hupomeinon heneka tōn ektos anoētos kai mōros dokōn; mēden boulou dokein epistasthai; kan doxēs tisi einai tis, apistei sautō.

If you wish to make progress, be willing, for the sake of external things, to be considered foolish and ignorant. Do not seek to appear knowledgeable. And if some people begin to think you are somebody, distrust that judgment yourself. Know this: it is not easy to keep your prohairesis — your power of rational choice — aligned with nature, and at the same time to pursue external things. To tend to one is necessarily to neglect the other.

— Enchiridion 13

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