《北史·列传·卷五十九》
文帝知道杨勇心中不安，在仁寿宫召见杨素，让他去观察杨勇的动静。杨素到了东宫，悄声无息地躲在外面，没有进去。杨勇穿戴整齐等待他，他就故意不进去，用以激怒杨勇。杨勇怨恨杨素失礼，怒形于色。
帝知其不安，在仁寿宫，使杨素观勇，素至东宫，偃息未入，勇束带待之，故亦不进以怒勇，勇衔之，形于言色。
Emperor Wen knew that Yang Yong was worried, and summoned Yang Su in Renshou Palace and asked him to observe Yang Yong's movements. Yang Su arrived at the East Palace, hid outside quietly, and did not go in. Yang Yong was dressed and waiting for him, and he deliberately did not go in, trying to provoke Yang Yong. Yang Yong resented Yang Su's rudeness and showed his anger.