calculus - $frac{f(t+T)}{f(T)} = f(t)$ then $f$ is an exponential function

If $f$ is a function such that for all $(T, t) \in \mathbb{R}^2_+$ we have :

$$\frac{f(t+T)}{f(T)} = f(t)$$

Then how can I prove that $f$ is an exponential function ?