"De Finibus Bonorum et Malorum"
While Cicero is an important ancient source on Epicureanism (he also wrote De Natura Deorum), 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).
Nevertheless, this work is an important source on ancient Epicureanism.
Read more about it on Wikipedia.