In Quantum Mechanics for Mathematicians by Leon A. Takhtajan, the author remarks on page 73:
According to von Neumann's theorem on a generating operator, for every commutative family $\mathbf{A}$ of self-adjoint operators (not necessarily finite) on a separable Hilbert space $\mathscr{H}$ there is a generating operator — a self-adjoint operator $R$ on $\mathscr{H}$ such that all operators in $\mathbf{A}$ are functions of $R$.
Here self-adjoint operators need not be bounded.
Where can I find a proof for this theorem? Thanks in advance!