Euripides, fr. 439 (Full text on the Scaife Viewer)
“Alas! I wish facts had a voice for people
So that clever speakers would be nothing.
Now instead men with turning tongues steal away
The truest things: and what should seem true cannot.”
φεῦ φεῦ, τὸ μὴ τὰ πράγματ’ ἀνθρώποις ἔχειν
φωνήν, ἵν’ ἦσαν μηδὲν οἱ δεινοὶ λέγειν.
νῦν δ’ εὐτρόχοισι στόμασι τἀληθέστατα
κλέπτουσιν, ὥστε μὴ δοκεῖν ἃ χρὴ δοκεῖν
This is from Euripides’ lost Hippolytus Veiled, a play in which the deception of Theseus results in the death of his son. His words sound idealist and almost noble, but as the story goes his wife Phaedra lies about sexual advances from her stepson Hippolytus and Theseus curses him. One of Theseus’ interlocutors replies (fr. 440):
“Theseus, I advise you that this is best, if you think through it:
Don’t ever believe that you hear the truth from a woman.”
Θησεῦ, παραινῶ σοὶ τὸ λῷστον, εἰ φρονεῖς,
γυναικὶ πείθου μηδὲ τἀληθῆ κλύων.
Nope. Not very nice, Euripides.