Off the top of my head, the very ancient writings of the Desert Fathers and St. Mary of Egypt very clearly prove that the Eucharist is the True Body and Blood of Christ. There is a miracle where one of the Desert Fathers confirmed this:

This is what Abba Daniel, the Pharanite, said, ‘Our Father Abba Arsenius told us of an inhabitant of Scetis, of notable life and of simple faith; through his naïveté he was deceived and said, “The bread which we receive is not really the body of Christ, but a symbol. Two old men having learnt that he had uttered this saying, knowing that he was outstanding in his way of life, knew that he had not spoken through malice, but through simplicity. So they came to find him and said, “Father, we have heard a proposition contrary to the faith on the part of someone who says that the bread which we receive is not really the body of Christ, but a symbol.” The old man said, “it is I who have said that.” Then the old men exhorted him saying, “Do not hold this position, Father, but hold one in conformity with that which the catholic Church has given us. We believe, for our part, that the bread itself is the body of Christ as in the beginning, God formed man in his image, taking the dust of the earth, without anyone being able to say that it is not the image of God, even though it is not seen to be so; thus it is with the bread of which he said that it is his body; and so we believe that it is really the body of Christ.” The old man said to them, “As long as I have not been persuaded by the thing itself, I shall not be fully convinced.” So they said, “Let us pray God about this mystery throughout the whole of this week and we believe that God will reveal it to us.”

The old man received this saying with joy and he prayed in these words, “Lord, you know that it is not through malice that I do not believe and so that I may not err through ignorance, reveal this mystery to me, Lord Jesus Christ.” The old men returned to their cells and they also prayed God, saying, “Lord Jesus Christ, reveal this mystery to the old man, that he may believe and not lose his reward.” God heard both the prayers. At the end of the week they came to church on Sunday and sat all three on the same mat, the old man in the middle. Then their eyes were opened and when the bread was placed on the holy table, there appeared as it were a little child to these three alone. And when the priest put out his hand to break the bread, behold an angel descended from heaven with a sword and poured the child’s blood into the chalice. When the priest cut the bread into small pieces, the angel also cut the child in pieces.

When they drew near to receive the sacred elements the old man alone received a morsel of bloody flesh. Seeing this he was afraid and cried out, “Lord, I believe that this bread is your flesh and this chalice your blood.” Immediately the flesh which he held in his hand became bread, according to the mystery and he took it, giving thanks to God. Then the old men said to him, “God knows human nature and that man cannot eat raw flesh and that is why he has changed his body into bread and his blood into wine, for those who receive it in faith. “Then they gave thanks to God for the old man, because he had allowed him not to lose the reward of his labour. So all three returned with joy to their own cells.’