《太平广记·卷二百四十八·诙谐四》
杨素是关中人，侯白是山东人。杨素经常突然发难，提出难题，想让侯白回答不上来。
素关中人，白山东人，素尝卒难之，欲其无对。
Yang Su was from Guanzhong, and Hou Bai was from Shandong. Yang Su often launched a surprise attack and raised a difficult question, trying to make Hou Bai unable to answer.