《六韬·龙韬·兵征》
凡是攻城围邑，如果过了十天仍不打雷下雨，必须迅速撤围，因为城中一定有贤能的辅助。这样，就可以知道为什么可攻就攻，不可攻就停止的道理了。 
凡攻城围邑，过旬不雷不雨，必亟去之，城必有大辅，此所以知可攻而攻，不可攻而止。 
When besieging a city, if there is no thunder or rain after ten days, we must withdraw immediately because there must be a capable and virtuous sage assisting the city. This is how we can understand why we should attack when we can and retreat when we cannot.