"The Lord Jesus, on the night he was handed over, took bread, and, after he had given thanks, broke it and said, 'This is my body that is for you. Do this in remembrance of me.' In the same way also the cup, after supper, saying, 'This cup is the new covenant in my blood. Do this, as often as you drink it, in remembrance of me'" (1 Cor 11.23-25).
The Didache - one of the earliest Church documents (A.D. 70, written while John was still living) - identifies the Eucharist as the sacrifice of Malachi 1.11.
"Take note of those who hold heterodox opinions on the grace of Jesus Christ which has come to us, and see how contrary their opinions are to the mind of God. ... They abstain from the Eucharist and from prayer because they do not confess that the Eucharist is the flesh of our Savior Jesus Christ, flesh which suffered for our sins and which that Father, in his goodness, raised up again. They who deny the gift of God are perishing in their disputes" (Ignatius of Antioch, "Letter to the Smyrnaeans" 6:2-7:1 [A.D. 110]).
St. Vincent of Lerins, a fifth-century French bishop writes, "[I]n the Catholic Church itself, all possible care must be taken, that we hold that faith which has been believed everywhere, always, by all. For that is truly and in the strictest sense 'Catholic,' which, as the name itself and the reason of the thing declare, comprehends all universally. This rule we shall observe if we follow universality, antiquity, consent. We shall follow universality if we confess that one faith to be true, which the whole Church throughout the world confesses; antiquity, if we in no wise depart from those interpretations which it is manifest were notoriously held by our holy ancestors and fathers; consent, in like manner, if in antiquity itself we adhere to the consentient definitions and determinations of all, or at the least of almost all priests and doctors" (Common, 2 [italics added]).