Laudator Temporis Acti

Athenaeus 14.623 f-824 a (tr. Charles Burton Gulick):

To quote The Harp-Singer of Theophilus: "A mighty treasure, good sirs, and a constant one, is music for all who have learned it and are educated." For indeed it trains character, and tames the hot-tempered and those whose opinions clash. The Pythagorean Cleinias, for example, as Chamaeleon of Pontus records, whose conduct and character were exemplary, would always take his lyre and play on it whenever it happened that he was exasperated to the point of anger. And in answer to those who inquired the reason he would say, "I am calming myself down."

μέγας γάρ, ὦ μακάριοι, κατὰ τὸν Θεοφίλου Κιθαρῳδόν,

θησαυρός ἐστιν καὶ βέβαιος μουσικὴ
ἅπασι τοῖς μαθοῦσι παιδευθεῖσί τε.

καὶ γὰρ τὰ ἤθη παιδεύει καὶ τοὺς θυμοειδεῖς καὶ τὰς γνώμας διαφόρους καταπραΰνει. Κλεινίας γοῦν ὁ Πυθαγόρειος, ὡς Χαμαιλέων ὁ Ποντικὸς ἱστορεῖ, καὶ τῷ βίῳ καὶ τοῖς ἤθεσιν διαφέρων, εἴ ποτε συνέβαινεν χαλεπαίνειν αὐτὸν δι᾽ ὀργήν, ἀναλαμβάνων τὴν λύραν ἐκιθάριζεν. πρὸς δὲ τοὺς ἐπιζητοῦντας τὴν αἰτίαν ἔλεγεν 'πραΰνομαι.'

See M.L. West, "Music Therapy in Antiquity" Hellenica: Selected Papers on Greek Literature and Thought, Vol. III: Philosophy, Music and Metre, Literary Byways, Varia (Oxford: Oxford University Press, 2013), pp. 245-261.