《周书·列传·卷三十一》
晚年喜好冲虚安静，只以领悟道家真义为事，对过去的著述全都加以删削，所以文笔大多不存。
晚年虚静，唯以体道会真为务。旧所制述，咸削其藁，故文笔多并不存。
In his old age, he favored quiet and solitude. He only cared about grasping the true meaning of Taoism and eliminated all his previous writings; therefore, most of his written works no longer exist.