CHORUS: Those whose cause is just will never lack
good arguments.

[ΧΟΡΟΣ: φεῦ φεῦ: βροτοῖσιν ὡς τὰ χρηστὰ πράγματα
χρηστῶν ἀφορμὰς ἐνδίδωσ᾽ ἀεὶ λόγων.]

Euripides (485?-406? BC) Greek tragic dramatist

Hecuba [Hekabe; Ἑκάβη], l. 1238ff (c. 424 BC) [tr. Arrowsmith (1958)]
    (Source)

Speaking of Hecuba's appeal to Agamemnon for justice. In some translations, this is marked for Coryphaeus (Chorus Leader)

(Source (Greek)). Alternate translations:

How doth a virtuous cause inspire the tongue
With virtuous language!
[tr. Wodhull (1809)]

Ah! Ah! How do good deeds ever supply to men the source of good words!
[tr. Edwards (1826)]

Lo, how the good cause giveth evermore
To men occasion for good argument.
[tr. Way (Loeb) (1894)]

Ah! how a good cause always affords men an opening for a good speech.
[tr. Coleridge (1938)]

Good words always start out from good deeds.
[tr. Harrison (2005)]

Did you hear that? See how a good cause gives mortals material for a good argument!
[tr. Theodoridis (2007)]

Just causes make fertile soil for strong arguments!
[tr. Karden/Street (2011)]