“"Lord Dong is not the only man of power in the empire."”
Response in 189 to threats by Dong Zhuo. After this, Yuan Shao left the capital and began organising a coalition against Dong. This confrontation marked the end of the uneasy peace between the two. Source: "Xiandi Chunqiu" (獻帝春秋), page 190 of Sanguo Zhi (三國志).