Chapter 6, Verses 7 and 8 - Sun Tzu

You can be sure of succeeding in your attacks if you only attack places which are undefended. You can ensure the safety of your defense if you only hold positions that cannot be attacked. Hence that general is skillful in attack whose opponent does not know what to defend; and he is skillful in defense whose opponent does not know what to attack.
~ from The Art of War, Giles translation ~

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