Council of Trent — Session XIII, Canon II

“If any one saith, that, in the sacred and holy sacrament of the Eucharist, the substance of the bread and wine remains conjointly with the body and blood of our Lord Jesus Christ, and denieth that wonderful and singular conversion of the whole substance of the bread into the Body, and of the whole substance of the wine into the Blood — the species only of the bread and wine remaining — which conversion indeed the Catholic Church most aptly calls Transubstantiation; let him be anathema.”