星星之火

Almost always paired with the second half: 星星之火，可以燎原 ('A single spark can start a prairie fire'). The phrase originates in the 尚书·盘庚 (Book of Documents) and was famously used as the title of Mao Zedong's 1930 essay, arguing that small revolutionary forces in rural China would eventually grow into a nationwide movement.

Use it to describe a small beginning with disproportionate downstream impact - a startup, a movement, a rumor. Never use it to mean a literal small fire.