Love at first sight
https://jakubmarian.com/at-first-sight-vs-at-the-first-sight-in-english/
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However, “at first sight” is much more commonly used in the idiom “love at first sight”, which means “an instantaneous/immediate attraction”. It is used without an article in this idiomatic meaning:
It was love at first sight. (correct)
It was love at the first sight. (unnatural)
“Love at first sight” is the most common way to employ this phrase, but “at first sight” can also be used in the sense of “based on the first impression”:
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