idiomatic expression vs saying

idiomatic expression and saying both are nouns.

Word	Noun	Adjective	Verb	Adverb
idiomatic expression	Yes	No	No	No
saying	Yes	No	No	No

As nouns, saying is a hypernym of idiomatic expression; that is, saying is a word with a broader meaning than idiomatic expression:

idiomatic expression: an expression whose meanings cannot be inferred from the meanings of the words that make it up
saying: a word or phrase that particular people use in particular situations

Other hypernyms of idiomatic expression include expression, locution.

idiomatic expression (noun)	saying (noun)
an expression whose meanings cannot be inferred from the meanings of the words that make it up	a word or phrase that particular people use in particular situations