《资治通鉴·晋纪·晋纪三十六》
姚兴写信给秃发檀说： 现在，我派遣齐难讨伐刘勃勃，我担心他向西逃跑，所以命令姚弼等人带兵在河西一带截击他们。 
兴遗傉檀书曰： 今遣齐难讨勃勃，恐其西逸，故令弼等于河西邀之。 
Yao Xing wrote to Tuoba Tan, saying: Now, I am sending Qi Nan to attack Liu Bobo. I am worried that he will flee west, so I ordered Yao Bi and others to lead troops to intercept them in the Hexi area.