Sunzi , Chapter Eight
九變 - Nine Transformations
故用兵之法,無恃其不來,恃吾有以待之;
無恃其不攻,恃吾有所不可攻也。
Gù yòng bīng zhī fǎ, wú shì qí bù lái, shì wú yǒu yǐ dài zhī; wú shì qí bù gōng, shì wú yǒu suǒ bùkě gōng yě.
Therefore, the principles of strategy are, do not rely on him not coming, rely on waiting, do not rely on him not attacking, rely on being unassailable.
Trans., Scott M. Rodell