Catechism of the Catholic Church §1391 — excerpt
Holy Communion augments our union with Christ. the principal fruit of receiving the Eucharist in Holy Communion is an intimate union with Christ Jesus. Indeed, the Lord said: "He who eats my flesh and drinks my blood abides in me, and I in him."
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: Fanqith, Syriac Office of Antioch, Vol. 1, Commun., 237 a-b