Zhège mínjiān gùshi kǒukǒu-xiāngchuán, liúchuán le jǐ bǎi nián.
This folk tale has been passed down by word of mouth for several hundred years.
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From 《云笈七签》 (Song dynasty Daoist anthology): 圣人秘之,不形文字,口口相传 - the sages kept it secret, not committing it to writing, passing it mouth to mouth. Originally about esoteric transmission; now generic for oral spread.