# The $\mathrm{Hom}(E_2,E_1)$-action on $\mathrm{Ext}^1(E_2,E_1)$ [closed]

Let $E_1$ and $E_2$ be objects in an abelian category $\mathcal{C}$. How can I see the natural action of $\mathrm{Hom}(E_2,E_1)$ on $\mathrm{Ext}^1(E_2,E_1)$?