Elliptic regularity of weak solutions

Question

I am looking for a precise reference for this result (I hope it is right).

If $u\in W^{1,2}(\mathbb{R}^n)$ weak solution to

$$\Delta u+b(x)\cdot Du+ c(x)u=0$$

where $b\in C^1$ and $c\in C^0$, then $u\in C^2(\mathbb{R}^N)$.

Thanks a lot.