Chapter 13, Verse 27 (The End) - Sun Tzu

Hence it is only the enlightened ruler and the wise general who will use the highest intelligence of the army for purposes of spying and thereby they achieve great results. Spies are a most important element in water, because on them depends an army's ability to move.
~ from The Art of War, Giles translation ~

Go here to read the introductory post to the verses of The Art of War.