Theodore (A.D. 405) - Catechetical Homilies 5:1:

"When [Christ] gave the bread he did not say, "This is the symbol of my body" but, "This is my body." In the same way when he gave the cup of his blood he did not say. "This is the symbol of my blood," but, "This is my blood," for he wanted us to look upon the [Eucharistic elements] after their reception of grace and the coming of the Holy Spirit not according to their nature, but receive them as they are, the body and blood of our Lord. We ought . . . not regard [the elements] merely as bread and cup) but as the body and blood of the Lord, into which they were transformed by the descent of the Holy Spirit"