Does any one know where one can find a reference about the following fact?


Let $X$ be a smooth projective variety over an algebraically closed field $k$. 
Fix two flat bundles $(L\_i,\nabla\_i)$ over $X$, (vector bundles with integrable connections) for $i=1,2$. 
Define a new flat bundle $(M,\nabla\_M)$ with $M= L\_2^\*\otimes L\_1$ and $\nabla\_M=\nabla\_2^\*\otimes \nabla\_1$.


Then all extension classes 
$$
0\to (L\_1,\nabla\_1)\to (H,\nabla)\to (L\_2, \nabla\_2)\to 0
$$ 
are described by the first deRham cohomology of $(M,\nabla\_M)$ (hypercohomology of the deRham complex).



reference of extension of flat bundle
You are looking at extensions of $L\_2$ by $L\_1$ as $\mathcal{D}\_X$-modules, which are classified by $\mathrm{Ext}^1\_{\mathcal{D}\_X}(L\_2,L\_1)\cong \mathrm{Ext}^1\_{\mathcal{D}\_X}(\mathcal{O}\_X,M)$. Now it is well-known in the theory of $\mathcal{D}$-modules that there is a canonical isomorphism of $R\mathcal{H}om\_{\mathcal{D}\_X}(\mathcal{O}\_X,M)$ with the de Rham complex $\Omega ^.\_X\otimes M$, so $\mathrm{Ext}^1\_{\mathcal{D}\_X}(\mathcal{O}\_X,M)$ is canonically isomorphic to $H^1\_{dR}(M)$.


