Enchiridion · §35

ὅταν τι κρίνας δέον ποιεῖν ποιῇς, μηδέποτε φύγῃς ὀφθῆναι ποιῶν αὐτό, κἂν ἄλλο τι ὑπολαμβάνωσιν οἱ πολλοί. εἰ μὲν γὰρ οὐκ ὀρθῶς ποιεῖς, αὐτὸ τὸ ἔργον φεῦγε· εἰ δὲ ὀρθῶς, τί δέδοικας τοὺς ἐπιπλήξοντας οὐκ ὀρθῶς;

hotan ti krinas deon poiein poiēs, mēdepote phugēs ophthēnai poiōn auto, kan allo ti hupolambanōsin hoi polloi. ei men gar ouk orthōs poieis, auto to ergon pheuge; ei de orthōs, ti dedoikas tous epiplēxontas ouk orthōs?

When you judge something to be the right thing to do and act on it, never avoid being seen doing it — even if most people will think badly of you for it. If the action is wrong, avoid the action itself; if it is right, why fear the criticism of people who are wrong to criticize it?

— Enchiridion 35

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