Good question, interesting to research. Short answer: Green got it from
Green says in his Author's Note:
I have gone on to make use of the pseudo-history of Geoffrey of Monmouth, and the verse chronicle of Layamon. These have given me a few ideas and details for Book One - but in essentials it is almost entirely Malory,...
I had previously checked that the detail the OP asked about was not in
Geoffrey of Monmouth's History of the Kings of Britain, so I turned to a modern English translation of Layamon's Brut which has this to say about that:
There Uther the king took Ygaerne for queen; Ygaerne was with child by Uther the king, all through Merlin's craft, before she was wedded. The time came that was chosen, then was Arthur born. So soon as he came on earth, elves took him; they enchanted the child with magic most strong, they gave him might to be the best of all knights; they gave him another thing, that he should be a rich king, they gave him the third, that he should live long; they gave to him the prince virtues most good, so that he was most generous of all men alive. This the elves gave him, and thus the child thrived. After Arthur, the blessed lady was born, she was named Anna, the blessed maiden; and afterwards she took (married) Loth, who possessed Leoneis (Lothian), she was in Leoneis lady of the people. Long lived Uther with mickle bliss here, with good peace, with much quiet, free in his kingdom.
The important bit here is that "the elves took him". This, I suppose is what Tennyson based his version of this detail in his Idylls of the King, which has passages like
Gareth was glad. Or if some other told,
How once the wandering forester at dawn,
Far over the blue tarns and hazy seas,
On Caer-Eryri's highest found the King,
A naked babe, of whom the Prophet spake,
'He passes to the Isle Avilion,
He passes and is healed and cannot die'—
Gareth was glad.
describing the scuttlebutt that Gareth heard in the scullery. Here "the Prophet" is Merlin. And elsewhere in the Idylls:
Then from the castle gateway by the chasm
Descending through the dismal night—a night
In which the bounds of heaven and earth were lost—
Beheld, so high upon the dreary deeps
It seemed in heaven, a ship, the shape thereof
A dragon winged, and all from stern to stern
Bright with a shining people on the decks,
And gone as soon as seen. And then the two
Dropt to the cove, and watched the great sea fall,
Wave after wave, each mightier than the last,
Till last, a ninth one, gathering half the deep
And full of voices, slowly rose and plunged
Roaring, and all the wave was in a flame:
And down the wave and in the flame was borne
A naked babe, and rode to Merlin's feet,
Who stoopt and caught the babe, and cried "The King!
Here is an heir for Uther!"
As to what the prophet said of the naked babe, look at The Legends of King Arthur and his Knights, 1862, by James Knowles (apparently with the complicity of Tennyson) with this version of Merlin's prophecy, paraphrased from Geoffrey.
When their battle was done, the king desired Merlin to tell him what it meant. Whereat he, bursting into tears, cried out this prophecy, which first foretold the coming of King Arthur.
“Woe to the red dragon, which figureth the British nation, for his banishment cometh quickly; his lurkingholes shall be seized by the white dragon—the Saxon whom thou, O king, hast called to the land. The mountains shall be levelled as the valleys, and the rivers of the valleys shall run blood; cities shall be burned, and churches laid in ruins; till at length the oppressed shall turn for a season and prevail against the strangers. For a Boar of Cornwall shall arise and rend them, and trample their necks beneath his feet. The island shall be subject to his power, and he shall take the forests of Gaul. The house of Romulus shall dread him—all the world shall fear him—and his end shall no man know; he shall be immortal in the mouths of the people, and his works shall be food to those that tell them.
Here "Boar of Cornwall" is Arthur.
So Layamon and Tennyson posit a different CV for Arthur than Malory does: childhood with the elves in Avalon, rather than with Sir Ector. Green uses elements of both.