Chapter 9, Verse 17 - Sun Tzu

If in the neighborhood of your camp there should be any hilly country, ponds surrounded by aquatic grass, hollow basins filled with reeds, or woods with thick undergrowth, they must be carefully routed out and searched; for these are places where men in ambush or insidious spies are likely to be lurking.
~ from The Art of War, Giles translation ~

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