Pindar, Pythian 3, 86-95
But an untroubled life did not abide with Aiakos' son Peleus
or with godlike Kadmos; yet they are said to have attained
the highest happiness of any men, for they even heard
the golden-crowned Muses singing on the mountain and
in seven-gated Thebes, when one married ox-eyed Harmonia,
the other Thetis, wise-counseling Nereus' famous daughter;
the gods feasted with both of them,
and they beheld the regal children of Kronos
on their golden thrones and received their wedding gifts.

(William H. Race, Pindar: Olympian Odes, Pythian Odes, Cambridge, Mass.-London: Harvard University Press and Heinemann, 1997: 255)

Plutarchian De Musica (On Music) 40, 1145e-1146a
'Homer, that splendid poet, taught us the uses of music that are appropriate for a man. [e] Thus to show that music is of value in many situations, his poem describes Achilles digesting his anger with the help of music which he had learned from the wise Cheiron: "They found him beguiling his heart with the clear-sounding phorminx. It was beautiful and skilfully decorated, and the crossbar on it was silver: he had chosen it from the spoils when he sacked the city of E๋tion. With it he was giving delight to his heart, and singing the famous deeds of men."
'"Notice," Homer is saying, "how music should be used, since it was suitable for Achilles, son of the most upright Peleus, to sing of the glories of men and the deeds of demigods." [f] Homer has also shown us the occasion which accords with its use, revealing it as a valuable and pleasant exercise for a man not actively occupied. Achilles was a man of war and action, but he was taking no part in the perils of war because of his anger with Agamemnon: hence Homer thought it suitable for the hero to sharpen his spirit with the noblest songs, so that he should be prepared to go out into battle, as he was soon to do; and this is plainly what he was doing as he recounted deeds of long ago.
'That is what the ancient music was like, [1146a] and what it was useful for. Thus we hear of Heracles, Achilles, and many others making use of music, and their teacher, according to tradition, was the wise Cheiron, who gave instruction not only in music but in justice and medicine as well.

(Barker 1984: 246-247)

Other literary sources:
Homer, Ilias, IX, 186-189
Diodorus Siculus V. 49
Plinius, Naturalis historia XXXVI. 29;
Philostratus, Heroicus 55. 3;
Philostratus, Imagines II. 2;
Pausanias 3. 18. 12;
scholia Hom. Ilias IX. 486 Erbse.

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