Cardinality of subset of the real numbers

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In a response I read, someone stated that any uncountable subset if the real numbers had the cardinality of the real numbers. Is this true and if so where can I find a reference to that result?

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This is the famous continuum hypothesis. Famous results by Cohen and Gödel imply that it is impossible to prove/disprove that there exists a cardinal between $\aleph_0$ and $2^{\aleph_0}$ within the framework of ZFC axioms.
There is also a well-known result that any uncountable Borel subset of $\mathbb{R}$ same cardinality as that of continuum. Maybe this was the result they had in mind.