三人成虎 (sān rén chéng hǔ): three men talking makes a tiger ; repeated rumor becomes a fact

三sān人rén成chéng虎hǔ

three men talking makes a tiger ; repeated rumor becomes a fact

One person reporting a tiger in the marketplace is not believed; three are. For the rumour that becomes fact purely by repetition.

A Chinese idiom (成语)