"De Natura Deorum"

On the Nature of the Gods by Cicero is written as a dialogue between Cicero and representatives from Epicurean, Stoic and Academic Sceptic schools. Book 1 contains the discourse of Velleius — a Senator and Epicurean — which consists of three parts: a general attack on Platonist and Stoic cosmology; a historical review of the earlier philosophers; and an exposition of Epicurean theology.

While Cicero is an important ancient source on Epicureanism (he also wrote De Finibus Bonorum et Malorum), he is not a sympathetic reporter — in fact “For the Epicurean philosophy Cicero had only disdain throughout most of his life, though his best friend Atticus was an Epicurean. This disdain leads him to seriously misrepresent its teachings…” (IEP).

Still, this is an important source of information on ancient Epicureanism. Read more about it on Wikipedia.