Euclidean vector space/Homotheties/Angle-preserving/Exercise

Let ${\displaystyle {}V}$ be a Euclidean vector space, and let ${\displaystyle {}\varphi \colon V\rightarrow V}$ denote a homothety with factor ${\displaystyle {}s\neq 0}$. Show that ${\displaystyle {}\varphi }$ is angle-preserving.