Catechism of the Catholic Church §1939 — excerpt
The principle of solidarity, also articulated in terms of "friendship" or "social charity," is a direct demand of human and Christian brotherhood.
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: John Paul II, SRS 38 40 · CA 10 · Pius XII, Summi pontificatus, October 20, 1939 · AAS 31 (1939) 423 ff