杯bēi弓gōng蛇shé影yǐng

Literal: the bow's reflection in the cup looks like a snake
Figurative: To be frightened by a false alarm; to let groundless suspicion make one anxious or even ill.

Where does 杯弓蛇影 come from?

A man invited to drink thought he saw a snake wriggling in his wine cup and, after drinking, fell ill with fright. Later the host realized that a bow hanging on the wall was reflected in the cup; when he showed the guest there was no snake at all, the man recovered at once. It was the fear, not any snake, that had sickened him.

Source (出处): 《晋书》

How is 杯弓蛇影 used? Two examples

听tīng到dào一yī点diǎn风fēng声shēng他tā就jiù草cǎo木mù皆jiē兵bīng，简jiǎn直zhí是shì杯bēi弓gōng蛇shé影yǐng。
At the slightest rumor he sees enemies everywhere; it is sheer fright at his own shadow.
医yī生shēng说shuō你nǐ身shēn体tǐ没méi问wèn题tí，别bié再zài杯bēi弓gōng蛇shé影yǐng，自zì己jǐ吓xià自zì己jǐ了le。
The doctor says you are fine; stop scaring yourself over imaginary threats.

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