《魏书·列传·卷八十四》
庾楷暗地里差遣使者私自与元显勾结，说桓玄已经大失人心，部下都不愿替他卖力，如果朝廷派兵征伐桓玄，自己愿意充当内应。
庾楷密使自结于元显，说玄大失人情，众不为用，若朝廷遣军。已当内应。
Yu Kai secretly sent an envoy to privately collude with Yuan Xian, saying that Huan Xuan had lost the hearts of the people and his subordinates were unwilling to work for him. If the court sent troops to conquer Huan Xuan, he would be willing to be the internal responder.