Catechism of the Catholic Church §2271 — excerpt
Since the first century the Church has affirmed the moral evil of every procured abortion. This teaching has not changed and remains unchangeable. Direct abortion, that is to say, abortion willed either as an end or a means, is gravely contrary to the moral law:
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: Didache 2, 2: SCh 248, 148 · Ep. Barnabae 19, 5: PG 2, 777 · Ad Diognetum 5, 6: PG 2, 1173 · Tertullian, Apol. 9: PL 1, 319-320 · GS 51 # 3