Snorri Sturluson: The Prose Edda
13th century, Icelandic: numerous copies in Iceland, Copenhagen.
Whenever any god or goddess felt old age creeping upon them, Idun would give them one of her apples to eat, and they would find themselves back in the prime of their youth.
The pagan Scandinavian goddess Idun kept some magic apples in a box, according to Snorri Sturluson, who in the 13th century was worried that the old pagan knowledge upon which traditional Icelandic poetry rested was about to die, and so he took the trouble to commit to writing all that he knew. Whenever any god or goddess felt old age creeping upon them, he recalled, Idun would give them one of her apples and immediately they would find themselves back in the prime of their youth.
One day, it is told, the god Loki allowed himself to be coerced into betraying the gods. He told Idun that he knew of another tree of magical apples and urged her to bring her box of apples to compare them with. Once out in the forest, she was abducted by a giant and she and her apples were taken to Giantland. As the gods became old and grey, and with the threat of infirmity hanging over them, they threatened Loki with such dire consequences if he did not recover the apples that Loki was forced to take on the shape of a falcon and fly to Giantland to retrieve them, and to rescue Idun.