Hello everyone, could anyone please tell me where can I find information about the Elton–Odell theorem?

It states:

For any infinite dimensional Banach space $X$ there is a $q > 1$ so that $X$ contains a sequence $(x_n)$ with $\|x_n\|=1$ and $\|x_n-x_m\|\ge q$, whenever $m\ne n$.

Thanks