Convexity inequality

Question

How can i, for a convex function such that $\nabla f$ is L-lipschitz, i.e $\|\nabla f(x) -\nabla f(y) \| \leq L\|x-y\|$, this inequality: \begin{equation} f(x)-f(y) \leq \langle \nabla f(x), x-y \rangle - \frac{1}{2L} \|\nabla f(x) - \nabla f(y) \|^2 \end{equation} Thank you in advance