She [Athena] threw her tasseled aegis about. her shoulders, wreathed round with Rout as with a fringe, and on it were Strife, and Strength, and Panic whose blood runs cold; moreover there was the head of the dread monster Gorgon, grim and awful to behold, portent of aegis-bearing Zeus (original Greek).
There [at Phlegra] the earth brought forth the Gorgon, a dreadful monster.
 As an ally for her children and trouble for the gods?
Yes; and Pallas, the daughter of Zeus, killed it.
[What fierce shape did it have?
A breastplate armed with coils of a viper.]
Is this the story which I have heard before?
 That Athena wore the hide on her breast.
And they call it the aegis, Pallas’ armor? (original Greek)
Hesiod, Th (Theogony) 270-81:
 And again, Ceto bore to Phorcys the fair-cheeked Graiae, sisters grey from their birth: and both deathless gods and men who walk on earth call them Graiae, Pemphredo well-clad, and saffron-robed Enyo, and the Gorgons who dwell beyond glorious Ocean  in the frontier land towards Night where are the clear-voiced Hesperides, Sthenno, and Euryale, and Medusa who suffered a woeful fate: she was mortal, but the two were undying and grew not old. With her lay the Dark-haired One in a soft meadow amid spring flowers.  And when Perseus cut off her head, there sprang forth great Chrysaor and the horse Pegasus (original Greek).
Aspis (or Shield of Herakles, part of Hesiodic corpus), 216-37:
There, too, was the son of rich-haired Danae, the horseman Perseus: his feet did not touch the shield and yet were not far from it—very marvellous to remark, since he was not supported anywhere; for so did the famous Lame One fashion him of gold with his hands.  On his feet he had winged sandals, and his black-sheathed sword was slung across his shoulders by a cross-belt of bronze. He was flying swift as thought. The head of a dreadful monster, the Gorgon, covered the broad of his back, and a bag of silver—a marvel to see—  contained it: and from the bag bright tassels of gold hung down. Upon the head of the hero lay the dread cap of Hades which had the awful gloom of night. Perseus himself, the son of Danae, was at full stretch, like one who hurries and shudders with horror. And after him rushed  the Gorgons, unapproachable and unspeakable, longing to seize him: as they trod upon the pale adamant, the shield rang sharp and clear with a loud clanging. Two serpents hung down at their girdles with heads curved forward:  their tongues were flickering, and their teeth gnashing with fury, and their eyes glaring fiercely (original Greek).
Paris, Louvre CA 795: relief pithos with Perseus and Medousa as Kentauros
Drawing from Daremberg and Saglio, Dicionnaire des antiquités grecques et romaines (1896 ff.), vol. 4.2, p. 403
Paris, Louvre CA 937: relief pithos fragment with Perseus
Wikimedia Commons image
Louvre (no image)
Eleusis, Archaeological Museum: Protoattic amphora with Gorgons and Perseus
Details of beheaded Medousa and her Gorgon sisters from pp. 324, 326 and 327, Kalliopi Papaggeli, Eleusis: The Archaeological Site and the Museum (2002)