I am seeking a reference that says:
If $V \subset H \subset V^*$ is a Gelfand triple with all spaces Hilbert spaces and if $V \subset H$ is a compact embedding, then there is a basis of $V$ which is orthogonal wrt. $V$ and orthonormal wrt. $H$.
I know how to prove but I just want something to cite. Anyone have any ideas? I tried the usual sources but had no luck.