$\mathbf{Cat}$ the category of the categories is a category

Question

I'm studying this book and I'm trying to prove this assertion the author made:

The identity functor is the identity in this category, i.e., for each category $C$, $Id_C:C\to C$ is the identity functor.

I'm having troubles to prove the associativity of the composition of functors.

Thanks in advance.