《资治通鉴·汉纪·汉纪七》
现在匈奴人经常在边界一带放牧、打猎，察看汉军守边士兵的状况，发现汉军人少，就会入侵。
今胡人数转牧、行猎于塞下，以候备塞之卒，卒少则入。
Now, the Xiongnu often herded their animals and hunted along the border, observing the conditions of the Han soldiers guarding the border; if they found that the Han soldiers were few in number, they would invade.