《后汉书·列传·袁绍刘表列传上》
许攸献计说： 曹操兵少而且全部人马用来抵挡我军，许县剩下的守卫力量必然空虚很弱。
许攸进曰： 曹操兵少而悉师拒我，许下余守势必空弱。
Xu You offered a plan: Cao Cao's army was small and all the troops were used to resist my army, so the remaining guards in Xuxian must be very weak.