Plato, Laws 656c

ὅπου δὴ νόμοι καλῶς εἰσι κείμενοι ἢ καὶ εἰς τὸν ἔπειτα χρόνον ἔσονται τὴν περὶ τὰς μούσας παιδείαν τε καὶ παιδιάν, οἰόμεθα ἐξέσεσθαι τοῖς ποιητικοῖς, ὅτιπερ ἂν αὐτὸν τὸν ποιητὴν ἐν τῇ ποιήσει τέρπῃ ῥυθμοῦ ἢ μέλους ἢ ῥήματος ἐχόμενον, τοῦτο διδάσκοντα καὶ τοὺς τῶν εὐνόμων παῖδας καὶ νέους ἐν τοῖς χοροῖς, ὅτι ἂν τύχῃ ἀπεργάζεσθαι πρὸς ἀρετὴν ἢ μοχθηρίαν; οὔτοι δὴ τοῦτό γε λόγον ἔχει· πῶς γὰρ ἄν;

Now where laws are, or will be in the future, rightly laid down regarding musical education and recreation, do we imagine that poets will be granted such licence that they may teach whatever form of rhythm or tune they best like themselves to the children of law-abiding citizens and the young men in the choirs, no matter what the result may be in the way of virtue or depravity? That would be unreasonable, most certainly.