《左传·成公·成公十年》
君子说： 忠诚是美德，所忠的人不合式尚且不可以，何况本人又不好呢？ 
君子曰： 忠为令德，非其人犹不可，况不令乎？ 
The gentleman said: Loyalty is a virtue, and when the object of loyalty is improper, it is not permissible. How much less proper when the man himself is not good.