St. Thomas Aquinas
“This sacrament is not only a sacrament, but also a sacrifice. For, it has the nature of a sacrifice inasmuch as in this sacrament Christ’s Passion is represented, whereby Christ ‘offered Himself a Victim to God’ (Eph. 5:2), and it has the nature of a sacrament inasmuch as invisible grace is bestowed in this sacrament under a visible species.”
Source: St. Thomas Aquinas, Summa Theologica