Catechism of the Catholic Church §1274 — excerpt
The Holy Spirit has marked us with the seal of the Lord ("Dominicus character") "for the day of redemption." "Baptism indeed is the seal of eternal life."
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: St. Augustine, Ep. 98, 5: PL 33, 362 · St. Irenaeus, Dem ap. 3: SCh 62, 32 · Roman Missal, EP I (Roman Canon) 97