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

He set a watch upon the Gladden Fields; but soon he discovered that the servants of Dol Guldur were searching all the ways of the River in that region. Then he perceived that Sauron also had learned of the manner of Isildur's end, and he grew afraid and withdrew to Isengard and fortified it; and ever he probed deeper into the lore of the Rings of Power and the art of their forging. But he spoke of none of this to the Council, hoping still that he might be the first to hear news of the Ring. He gathered a great host of spies, and many of these were birds; for Radagast lent him his aid, divining naught of his treachery, and deeming that this was but part of the watch upon the Enemy.