《隋书·列传·卷十》
杨素到了东宫，在外屋歇息不进杨勇里屋，杨勇系好腰带等待他，杨素有意长时间不进去，来激怒杨勇。
素至东宫，偃息未入，勇束带待之，故久不进，以激怒勇。
Yang Su arrived at the East Palace and rested in the outer room but didn't enter Yang Yong's inner room. Yang Yong fastened his belt waiting for him. Yang Su intended to deliberately not go in for a long time to irritate Yang Yong.