Catechism of the Catholic Church §1706 — excerpt
By his reason, man recognizes the voice of God which urges him "to do what is good and avoid what is evil." Everyone is obliged to follow this law, which makes itself heard in conscience and is fulfilled in the love of God and of neighbor.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: GS 16