From Ovid's "Metamorphoses", Book XI:

Old Proteus once had said to Thetis, "Bear
A child, fair goddess of the waves. For you
Shall be the mother of a youth whose deeds
In his brave years of manhood shall surpass
His father's and he'll win a greater name."
Therefore, for fear the world might ever have
A greater than himself, Jove shunned the bed
Of Thetis, fair see-goddess, though his heart
Was fired with no cool flame, and in his place
As lover bade his grandson Peleus take
In his embrace the virgin of the waves.

There is a curving bay in Thessaly
Shaped like a sickle; two long arms run out
And were the water deeper, there would be
A harbour. Smooth across the shallow sand
The sea extends; the shore is firm; it holds
No footprints, shows no passage, slopes unlined
By seaweed. Myrtles grow near by, a grove
Of double-coloured berries. In their midst
There lies a grotto, formed maybe by art,
Maybe by nature, rather though by art,
Where Thetis used to come, naked, astride
Her bridled dolphin. There, as she lay lapped
In sleep, Peleus surprised her and, his fond
Entreaties all repulsed, assaulted her
Winding his two strong arms around her neck.
And had she not resorted to her arts
And changed her shape so often, he'd have gained
The end he dared. But first she was a bird -
That bird he held; and then a sturdy tree -
That tree he fastened on; her third shape was
A stripy tigress - Peleus, terrified,
released his hold on her and let her go.
He prayed then to the sea-gods, offering wine
Poured on the water, smoke of incense, flesh
Of sheep, till Proteus from his briny deep
Said "Peleus, you shall gain the bride you seek
If, while she's sleeping in her rocky cave,
You catch her off her guard and truss her tight
With ropes that won't give way and, though she takes
A hundred spurious shapes, don't be deceived
But grapple it, whatever it is, until
She forms again the shape she had before."
So Proteus spoke and sank into the sea,
His wavelets washing over his last words.

The sun was setting and his chariot
Sloped to the western waves, when the fair child
Of Nereus sought the grotto and resumed
Her usual couch. Peleus had barely touched
Her lovely limbs before from shape to shape
She changed, until she felt her body trussed,
Her arms pinioned apart. And then at last,
Sighing "With some god's help", she said "you've won".
And there revealed stood... Thetis. Self-confessed
He held her, hopes triumphant, to his side
And filled with great Achilles his fair bride.