三sān人rén成chéng虎hǔ

Literal: three people make a tiger
Figurative: A rumor or lie, repeated by enough people, ends up being taken as truth. It warns that collective insistence can make a falsehood prevail over reality.

Where does 三人成虎 come from?

The minister Pang Cong of the state of Wei was about to leave for Zhao as a hostage, escorting the crown prince. Before departing he asked the king of Wei: "If one person said there was a tiger in the marketplace, would you believe it?" The king said no. "And if two people said it?" The king began to waver. "And if three said it?" Then the king said he would believe it. Pang Cong pointed out that everyone knows there are no tigers in a market, yet three voices were enough to conjure one; since he was going far away and many would speak ill of him behind his back, he begged the king not to trust slander. The king promised he would not, but by the time Pang Cong returned the gossip had done its work, and he never regained the king's audience.

Source (出处): 《战国策·魏策》

How is 三人成虎 used? Two examples

网wǎng上shàng的de谣yáo言yán传chuán得de很hěn快kuài，三sān人rén成chéng虎hǔ，很hěn多duō人rén就jiù真zhēn的de信xìn了le。
Rumors spread very fast online; repeated enough times, many people end up actually believing them.
别bié听tīng他tā们men乱luàn说shuō，三sān人rén成chéng虎hǔ，假jiǎ的de也yě会huì被bèi当dàng成chéng真zhēn的de。
Don't listen to their loose talk; said often enough, even a lie gets taken as the truth.

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