Once a rumor spreads like wildfire, it's very hard to control.
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Comes from 《尚书·盘庚上》 (the Book of Documents): 若火之燎于原，不可向迩 ('like fire blazing across the plain, none can approach it'). The full chengyu 星星之火，可以燎原 (xīngxīng zhī huǒ, kěyǐ liáoyuán, 'a single spark can start a prairie fire') was made famous by Mao Zedong in his 1930 essay of the same name.
memory
燎 (liáo, 'to blaze across') + 原 (yuán, 'open plain') paints the image directly - fire racing across the steppe.
