《汉书·传·傅常郑甘陈段传》
单于派使者来问汉朝军队为何而来，答道： 单于上书说现在处境困难，愿意归附汉朝，亲自入汉朝朝见。
单于遣使问： 汉兵何以来？ 应曰： 单于上书言居困厄，愿归计强汉，身入朝见。
The Chanyu sent an envoy to ask the Han army why they had come. They answered: The Chanyu wrote a document saying that he is currently in a difficult situation and wishes to submit to the Han dynasty and visit the Han in person.