Since then [Jesus] Himself declared and said of the Bread, “This is My Body”, who shall dare to doubt any longer [that it is His Body]? And since He has Himself affirmed and said, “This is My Blood”, who shall ever hesitate, saying, that it is not His blood?
St. Cyril of Jerusalem, from Catechetical Lecture 22 written in the 4th century A.D.
See also 1 Corinthians 11:23-25, Matthew 26:26-28, Mark 14:22-24, Luke 22:19-20.
23For I have received of the Lord that which also I delivered unto you, that the Lord Jesus, the same night in which he was betrayed, took bread.
24And giving thanks, broke, and said: Take ye, and eat: this is my body, which shall be delivered for you: this do for the commemoration of me.
25In like manner also the chalice, after he had supped, saying: This chalice is the new testament in my blood: this do ye, as often as you shall drink, for the commemoration of me.
26And whilst they were at supper, Jesus took bread, and blessed, and broke: and gave to his disciples, and said: Take ye, and eat. This is my body.
27And taking the chalice, he gave thanks, and gave to them, saying: Drink ye all of this.
28For this is my blood of the new testament, which shall be shed for many unto remission of sins.
22And whilst they were eating, Jesus took bread; and blessing, broke, and gave to them, and said: Take ye. This is my body.
23And having taken the chalice, giving thanks, he gave it to them. And they all drank of it.
24And he said to them: This is my blood of the new testament, which shall be shed for many.
19And taking bread, he gave thanks, and brake; and gave to them, saying: This is my body, which is given for you. Do this for a commemoration of me.
20In like manner the chalice also, after he had supped, saying: This is the chalice, the new testament in my blood, which shall be shed for you.