people will believe a falsehood if many repeat it; argumentum ad populum
Proverb meaning that a lie repeated often enough can be taken as truth. Literally 'there is a tiger in the market'.
The proverb 'there is a tiger in the market' means that if many people say something, even a lie can become truth.
From a classical Chinese story in which a rumor of a tiger in the marketplace, repeated by many, eventually convinces the king. The exact historical derivation is uncertain.