§ Of the Rings of Power and the Third Age The Silmarillion

Thus it was that in two domains the bliss and beauty of the Elves remained still undiminished while that Age endured: in Imladris; and in Lothlórien, the hidden land between Celebrant and Anduin, where the trees bore flowers of gold and no Orc or evil thing dared ever come. Yet many voices were heard among the Elves foreboding that, if Sauron should come again, then either he would find the Ruling Ring that was lost, or at the best his enemies would discover it and destroy it; but in either chance the powers of the Three must then fail and all things maintained by them must fade, and so the Elves should pass into the twilight and the Dominion of Men begin.