杯弓蛇影

From a story in the Book of Jin:

樂廣嘗有親客久闕不復來，廣問其故，答曰：「前在坐，蒙賜酒，方欲飲，見杯中有蛇，意甚惡之，既飲而疾作。」時河南廳事壁上有角漆畫作蛇，廣意杯中蛇即角影也。復置酒於前處，謂客曰：「酒中復有所見不？」答曰：「所見如初。」廣乃告其所以。豁然意解，沉痾頓愈。 [Classical Chinese, trad.][▼ expand/hide]

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乐广尝有亲客久阙不复来，广问其故，答曰：“前在坐，蒙赐酒，方欲饮，见杯中有蛇，意甚恶之，既饮而疾作。”时河南厅事壁上有角漆画作蛇，广意杯中蛇即角影也。复置酒于前处，谓客曰：“酒中复有所见不？”答曰：“所见如初。”广乃告其所以。豁然意解，沉疴顿愈。 [Classical Chinese, simp.]

From: 648 CE, the Book of Jin, Chapter 43, translation from 100 Chinese Idioms and Their Stories

Yuè Guǎng cháng yǒu qīnkè jiǔ quē bù fù lái, Guǎng wèn qí gù, dá yuē: “Qián zài zuò, méng cì jiǔ, fāng yù yǐn, jiàn bēi zhōng yǒu shé, yì shèn wù zhī, jì yǐn ér jí zuò.” Shí Hénán tīngshì bì shàng yǒu jiǎo qī huà zuò shé, Guǎng yì bēi zhōng shé jí jiǎo yǐng yě. Fù zhì jiǔ yú qián chù, wèi kè yuē: “Jiǔ zhōng fù yǒu suǒjiàn bù?” Dá yuē: “Suǒjiàn rú chū.” Guǎng nǎi gào qí suǒyǐ. Huòrán yì jiě, chénkē dùn yù. [Pinyin]

Yue Guang had a frequent guest, who used to call on him regularly but had failed to turn up for some time. When Yue asked the reason why, the man answered: “On my last visit, you kindly treated me to wine. Just as I was going to take a sip, I saw a snake wriggling in the goblet and I felt quite sick. Nevertheless, I drank; then I soon fell ill.”
Now in those days it was a custom prevailing in Henan Prefecture for a prefect to have a bow painted to look like a snake and hung in his hall. Suspecting that the snake alluded to was nothing but a reflection of the decorated bow, he invited the man again for a drink and set the table in the usual spot.
“Do you still see something in the wine cup?” asked the host.
“Yes, exactly as before.” was the answer.
Then Yue Guang explained what he thought the whole trouble must have been. At that, the guest realized what had happened and instantly relieved of his obsession, and returned to normal.

杯弓蛇影

1. (figuratively) false alarm; self-created suspicion; atmosphere of fear and admonition

• 疑神疑鬼 (yíshényíguǐ)