Let $\mathcal{O}_K$ be the ring of integers in an algebraic number field $K$ and let $I \subset \mathcal{O}_K$ be a nonzero proper ideal.  It is not hard to see that the map $\text{SL}_n(\mathcal{O}_K) \rightarrow \text{SL}_n(\mathcal{O}_K/I)$ is surjective.  For instance, $\mathcal{O}_K/I$ is a product of local rings, and over such rings the special linear group is generated by elementary matrices.  I need this theorem in a paper I am writing, and I'd rather not spend a paragraph proving it.  Does anyone know a good reference?