そうぼとうちょ

a lie becomes the truth once enough people believe it

曾母、杼を投ず。Zengzhi(曾) was visiting a town where there happened to live a man with the same name. This other 'Zhengzhi' killed a man, and people mistakenly started telling the original Zhegzhi's mother母 that her son was a murderer. She didn't believe it at first, but after the third person told her, she threw投 aside her loom杼 in despair