《论语·为政篇》
等他回去后，我观察他私下里同别人讨论时，却能发挥我所讲的，可见颜回他并不愚笨呀！ 
退而省其私，亦足以发，回也不愚。 
When he came to me, however, he would discuss the lessons with me and demonstrate his grasp of them. And so I believe that he is not a stupid fellow.