Whence can I reference the following fact (I have seen it quoted as `standard' in respectable places, so I hope it is so)?:

Let $f : B_2(0) \to \mathbb{R}$, say $f \in L^2(B_2(0))$ . Suppose that there exists $\alpha \in (0,1)$ and $c > 0$ such that the following is true: For every $y \in B_1(0)$, there exists an affine function $l_y : \mathbb{R}^n \to \mathbb{R}$ such that

$\rho^{-n-2}\int_{B_{\rho}(y)}|f(x) - l_y(x)|^2dx \leq c\rho^{2\alpha}\int_{B_2(0)}|f(x)|^2dx$

for all $\rho \in (0,1/4)$.

Then $f \in C^{1,\alpha}(B_1(0))$ with $\|f\|_{C^{1,\alpha}(B_1(0))} \leq c\|f\|_{L^2(B_2(0))}$.