世外桃源 comes from
桃花源记, 'The Peach Blossom Spring', a prose piece by
陶渊明 (365 to 427 AD). In it, a fisherman stumbles upon a hidden valley of peach trees where people live in perfect, timeless harmony, untouched by the outside world. He cannot find it again when he returns. The story gave rise to the expression
世外桃源 as the Chinese equivalent of utopia.