Pindar, Olympian Ode 3. 12 ff :

“[The Olympic Games,] rites long years ago established by Herakles, set on his brow aloft that shining glory, wreathed upon his hair, of the green olive leaf; which once from Istros’ [the Danube’s] shady streams Amphitryon’s son brought hither, to be the fairest emblem of Olympia’s Games.For the Hyperborean folk, Apollon’s servants, he so persuaded with fair words, when, for the all-hospitable grove of Zeus, his loyal heart begged for the tree, to make shade for all men to share, and for brave deeds of valorous spirits, a crown. For he had seen long since his father’s [Zeus’] altars sanctified, and the light of evening smiling at mid-month to the golden care of the full-orbèd moon; and of the great Games had set up the contest and sacred judgment, with the rites of the four-yearly feast, on the high banks of Alpheios’ holy river. But the land of Pelops, and the vales by Kronos’ hill nourished no lovely trees, and his eyes saw a garden spread defenceless beneath the fierce rays of the sun.Then at length did his heart bid him be one, to journey to the land of Istria, where, long since, Leto’s daughter [Artemis], lover of horsemanship, received him. For he came from Arkadia’s high peaks and winding glens, by constraint of his father, to perform the bidding of Eurystheus, and bring back the Hind of the golden horns . . . And in that search he saw, too, the famed land that lay behind cold Boreas of bleak and frozen breath; and standing there marvelled to see the trees. And in his heart a dear resolve was born, to set them planted there, where ends the course twelve times encircled by the racing steeds.”