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

Now by fortune and his vigilance Mithrandir first learned of the Ring, ere Sauron had news of it; yet he was dismayed and in doubt. For too great was the evil power of this thing for any of the Wise to wield, unless like Curunír he wished himself to become a tyrant and a dark lord in his turn; but neither could it be concealed from Sauron for ever, nor could it be unmade by the craft of the Elves. Therefore with the help of the Dúnedain of the North Mithrandir set a watch upon the land of the Periannath and bided his time. But Sauron had many ears, and soon he heard rumour of the One Ring, which above all things he desired, and he sent forth the Nazgûl to take it. Then war was kindled, and in battle with Sauron the Third Age ended even as it had begun.