Greek Mythology

Zeus and Hermes visit the earth in search of genuine goodness, without revealing themselves. They search for many days, and are eventually weary, and an old couple spot this and invite them into their tiny, poor shack by a lake. They give generously of all the food they own, going without themselves in order to satisfy their guests' need. And give up their own bed by the fire so that their guests may sleep in comfort. In the morning, they wave the two on their way, never having known who they were.

Later, Zeus and Hermes return, and reveal themselves, and grant the couple a wish. They wish to die at the same time as each other, as neither could imagine living after the other was gone. Their wish is granted, and when they die, the gods transform then into an intertwining pair of tall trees, to watch quietly together over the lake they both loved in life.

I couldn't remember their names, but a quick Google reminded me: Baucis and Philemon