why is the dual of abelian discrete group the collection of irreducible representations of $G$ on the Hilbert space $\mathbb{C}$

Question

Let $G$ be a discrete abelian group, and consider its dual $\hat{G}$ i.e. the set of group homomorphisms from $G$ into $\mathbb{T}$. I am a bit confused that $\hat{G}$ is the set of irreducible representations of $G$ on the Hilbert space $\mathbb{C}$. Can someone supply reasoning or source that I can look at. Context: I am studying $C^*$-algebras.