Enchiridion · §23

ἐὰν ποτέ σοι γένηται ἔξω στραφῆναι πρὸς τὸ θέλειν ἀρέσαι τινί, ἴσθι ὅτι ἀπώλεσας τὴν ἔνστασιν. ἀρκοῦ οὖν ἐν παντὶ τῷ εἶναι φιλόσοφος· εἰ δὲ καὶ δοκεῖν βούλει, σαυτῷ φαίνου καὶ ἱκανός ἔσῃ.

ean pote soi genētai exō straphēnai pros to thelein aresai tini, isthi hoti apōlesas tēn enstasin. arkou oun en panti tō einai philosophos; ei de kai dokein boulei, sautō phainou kai hikanos esē.

If you ever turn outward to please someone, know that you have lost your footing. Be content therefore in every matter to be a philosopher in reality; and if you also wish to appear one, appear so to yourself — and that will be enough.

— Enchiridion 23

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