《资治通鉴·汉纪·汉纪五十五》
冀州别驾田丰劝袁绍说： 曹操与刘备交战，不会立即分出胜负，将军率军袭击他的后方，可以一举成功。 
冀州别驾田丰说袁绍曰： 曹操与刘备连兵，未可卒解。公举军而袭其后，可一往而定。 
The assistant registrar of Jizhou, Tian Feng, advised Yuan Shao: Cao Cao and Liu Bei are fighting, and will not immediately see who wins. General, if you lead your army to attack his rear, you can succeed with a single move.