In the paper by Akos Magyar, *Diophantine Equations and Ergodic Theorems*, one states in page 923 the following theorem:

Theorem 1: Let $Q(m)$ be a nondegenerate polynomial and $\Lambda$ is corresponding set of regular values. Then for a test function $\phi(x)\in\mathcal{S}(\mathbb{R}^n)$ one has: $$\lim_{\lambda\in\Lambda,\lambda\to\infty}\frac{1}{r_Q(\lambda)}\sum_{Q(m)=\lambda}\phi(\lambda^{-1/d}m)=\int_{Q(x)=1}\phi(x)d\sigma_Q(x),$$ where $r_Q(\lambda)= |\{m\in\mathbb{Z}^n : Q(m) = \lambda\}|$ is the number of solutions $m\in\mathbb{Z}^n$ of the diophantine equation $Q(m)=\lambda$.

That is, when the solution sets $Q(m)=\lambda$ are projected to the unit surface $Q(x)=1$ via the dilations $m\to\lambda^{-1/d}m$, they weakly converge to the surface measure $\frac{dS_Q(x)}{|Q'(x)|}$. This is well-known in the case $Q(x)$ is a quadratic form. My request is: Can anyone indicate me some references containing this well-known result and its proof in the case $Q(x)$ is a quadratic form?