Plato, Laws 812c

τε τοὺς ῥυθμοὺς καὶ τὰς τῶν ἁρμονιῶν συστάσεις, ἵνα τὴν τῶν μελῶν μίμησιν τὴν εὖ καὶ τὴν κακῶς μεμιμημένην, ἐν τοῖς παθήμασιν ὅταν ψυχὴ γίγνηται, τά τε τῆς ἀγαθῆς ὁμοιώματα καὶ τὰ τῆς ἐναντίας ἐκλέξασθαι δυνατὸς ὤν τις, τὰ μὲν ἀποβάλλῃ, τὰ δὲ προφέρων εἰς μέσον ὑμνῇ καὶ ἐπᾴδῃ ταῖς τῶν νέων ψυχαῖς, προκαλούμενος ἑκάστους εἰς ἀρετῆς ἕπεσθαι κτῆσιν συνακολουθοῦντας διὰ τῶν μιμήσεων. ἀληθέστατα λέγεις.

regarding rhythms and harmonic compositions, in order that when dealing with musical representations of a good kind or a bad, by which the soul is emotionally affected, they may be able to pick out the reproductions of the good kind and of the bad, and having rejected the latter, may produce the other in public, and charm the souls of the children by singing them, and so challenge them all to accompany them in acquiring virtue by means of these representations. Very true.