《资治通鉴·汉纪·汉纪二十九》
五威将帅说： 从前，中国对匈奴有四项约束，不可以接受逃亡投降的乌桓人，速送他们回去！ 
将帅曰： 前封四条，不得受乌桓降者。
The Five Mighty Generals and Commanders said: In the past, China had four restrictions on the Xiongnu, you cannot accept fleeing Wuhuan people who surrender, send them back quickly!