Dionysius of Halicarnassus, Lysias (§2):
Lysias is totally pure in his mode of expression, and is a regular paragon of the Attic dialect – not the archaic one used by Plato and Thucydides, but the one which was common during his own time, which can be seen in the speeches of Andocides, Critias, and a bunch of others. In this respect, which is indeed the chief and most important thing in speech writing (I am speaking of his ability to purify his dialect), no one among succeeding generations ever surpassed him, and indeed many people had not even the power to imitate him, with the exception of Isocrates. For he seems to me to have been the purest among the rest in his use of language among the successors of Lysias. I find this to be the one virtue worthy of study and imitation for an orator, and I would implore those who want to write or speak clearly to hold up Lysias as the perfect paradigm of that particular excellence.
καθαρός ἐστι τὴν ἑρμηνείαν πάνυ καὶ τῆς ᾿Αττικῆς γλώττης ἄριστος κανών, οὐ τῆς ἀρχαίας, ᾗ κέχρηται Πλάτων τε καὶ Θουκυδίδης, ἀλλὰ τῆς κατ’ ἐκεῖνον τὸν χρόνον ἐπιχωριαζούσης, ὡς ἔστι τεκμήρασθαι τοῖς τε ᾿Ανδοκίδου λόγοις καὶ τοῖς Κριτίου καὶ ἄλλοις συχνοῖς. κατὰ τοῦτο μὲν δὴ τὸ μέρος, ὅ πέρ ἐστι πρῶτόν τε καὶ κυριώτατον ἐν λόγοις, λέγω δὲ τὸ καθαρεύειν τὴν διάλεκτον, οὐθεὶς τῶν μεταγενεστέρων αὐτὸν ὑπερεβάλετο, ἀλλ’ οὐδὲ μιμήσασθαι πολλοὶ δύναμιν ἔσχον ὅτι μὴ μόνος ᾿Ισοκράτης· καθαρώτατος γὰρ δὴ τῶν ἄλλων μετὰ Λυσίαν ἐν τοῖς ὀνόμασιν οὗτος ἔμοιγε δοκεῖ γενέσθαι ὁ ἀνήρ. μίαν μὲν δὴ ταύτην ἀρετὴν ἀξίαν ζήλου καὶ μιμήσεως εὑρίσκω παρὰ τῷ ῥήτορι καὶ παρακελευσαίμην ἂν τοῖς βουλομένοις καθαρῶς γράφειν ἢ λέγειν ἐκεῖνον τὸν ἄνδρα ποιεῖσθαι παράδειγμα ταύτης τῆς ἀρετῆς.