Enchiridion · §28

εἰ μέν τις σώματί σου παρέδωκε τὸ σῶμα τῷ τυχόντι, ἠγανάκτεις ἄν· ὅτι δὲ τὴν γνώμην σου παρέδωκας τῷ τυχόντι, ἵνα ἐὰν λοιδορηθῇ ταραχθῇ καὶ σκυθρωπάσῃ, οὐκ αἰσχύνῃ τούτου;

ei men tis sōmati sou paredōke to sōma tō tuchonti, ēganakteis an; hoti de tēn gnōmēn sou paredōkas tō tuchonti, hina ean loidorēthē tarachthē kai skuthrōpasē, ouk aischunē toutou?

If someone handed your body over to any random passerby to do with as they pleased, you would be outraged. Yet you hand over your own mind to anyone who insults you — letting them disturb you and make you miserable — and you feel no shame about it.

— Enchiridion 28

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