It's easy to find Ramanujan's proof of Ramanujan primes:

Wikipedia mentions that Paul Erdős also had a proof:

Wikipedia article on Bertrand's Postulate

Does anyone know the full citation for Erdős's proof that for any number $n$, there exists a prime $p$ such that for all $x \ge p$, there are $n$ primes between $x$ and $2x$?

Thanks very much,

-Larry