Let $\mathcal{M}, \mathcal{N}$ be two Riemannian manifods. Suppose that $\mathcal{N}$ is properly and isometrically embedded in $\mathbb{R}^n$. The space of Sobolev maps between $\mathcal{M}$ and $\mathcal{N}$ is defined as follows \begin{equation} W^{1,2}(\mathcal{M}, \mathcal{N})=\{ u \in W^{1,2}(\mathcal{M}, \mathbb{R}^n)\mid u(x) \in \mathcal{N}\, a.e.\}. \end{equation}

Many papers say that $W^{1,2}(\mathcal{M}, \mathcal{N})$ is a Banach manifold, but no one gives a reference about how to prove this result. Can anybody give me some references?