Let $D=\mathbb{R}$ or $D=\mathbb{C} .$ Let $C \in \mathbb{C}$ be a constant and $f: D \rightarrow \mathbb{C}$ differentiable with
$$
f^{\prime}(z)=C f(z) \quad \text { for all } \quad z \in D
$$
If $A=f(0)$, then
$$
f(z)=A \exp (C z) \quad \text { for all } \quad z \in D
$$