The Mabinogion

And lo, there came four-and-twenty asses with their burdens of gold and of silver, and a tired way-worn man with each of them, bringing tribute to Arthur from the Islands of Greece. Then Kadyriaith the son of Saidi besought that a truce might be granted to Osla Gyllellvawr for the space of a fortnight and a month, and that the asses and the burdens they carried might be given to the bards, to be to them as the reward for their stay and that their verse might be recompensed during the time of the truce. And thus it was settled.

Then Kai arose, and he said, “Whosoever will follow Arthur, let him be with him to-night in Cornwall, and whosoever will not, let him be opposed to Arthur even during the truce.” And through the greatness of the tumult that ensued, Rhonabwy awoke. And when he awoke he was upon the yellow calf-skin, having slept three nights and three days.

And this tale is called the Dream of Rhonabwy. And this is the reason that no one knows the dream without a book, neither bard nor gifted seer; because of the various colours that were upon the horses, and the many wondrous colours of the arms and of the panoply, and of the precious scarfs, and of the virtue-bearing stones.