《史记·七十列传·平津侯主父列传》
那匈奴的习性，像群兽聚积和众鸟飞散一样，追赶他们就像捕捉影子一样。
夫匈奴之性，兽聚而鸟散，从之如搏影。
"The habits of the Xiongnu are like those of a herd of animals gathering together and birds scattering into flight; pursuing them would be like trying to grasp a shadow.