Minkowski space/Observer vector/Estimate/Exercise

Let ${\displaystyle {}V}$ be a Minkowski space with the Minkowski form ${\displaystyle {}\left\langle -,-\right\rangle }$, and let ${\displaystyle {}v,w}$ denote timelike vectors. Show the estimate

${\displaystyle {}\left\langle v,w\right\rangle ^{2}\geq \left\langle v,v\right\rangle \cdot \left\langle w,w\right\rangle \,.}$