Chapter 13, Verse 20 - Sun Tzu

Whether the object be to crush an army, to storm a city, or to assassinate an individual, it is always necessary to begin by finding out the names of the attendants, the aides-de-camp, and door-keepers and sentries of the general in command. Our spies must be commissioned to ascertain these.
~ from The Art of War, Giles translation ~

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