Apparently, one may argue that Justin Martyr was the first apart from the Bible account, but on his heels were the martyrs of Lyons. First is the quote and then commentary (emphasis mine).
And this food is called among us Εὐχαριστία1910 [the Eucharist], of which no one is allowed to partake but the man who believes that the things which we teach are true, and who has been washed with the washing that is for the remission of sins, and unto regeneration, and who is so living as Christ has enjoined. For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Saviour, having been made flesh by the Word of God, had both flesh and blood for our salvation, so likewise have we been taught that the food which is blessed by the prayer of His word, and from which our blood and flesh by transmutation are nourished, is the flesh and blood of that Jesus who was made flesh.1911 For the apostles, in the memoirs composed by them, which are called Gospels, have thus delivered unto us what was enjoined upon them; that Jesus took bread, and when He had given thanks, said, “This do ye in remembrance of Me,1912 this is My body;” and that, after the same manner, having taken the cup and given thanks, He said, “This is My blood;” and gave it to them alone. Which the wicked devils have imitated in the mysteries of Mithras, commanding the same thing to be done. For, that bread and a cup of water are placed with certain incantations in the mystic rites of one who is being initiated, you either know or can learn.
-Justin Martyr The First Apology Chapter LXVI-
1911 This passage is claimed alike by Calvinists, Lutherans, and Romanists; and, indeed, the language is so inexact, that each party may plausibly maintain that their own opinion is advocated by it. [But the same might be said of the words of our Lord himself; and, if such widely separated Christians can all adopt this passage, who can be sorry?] The expression, “the prayer of His word,” or of the word we have from Him, seems to signify the prayer pronounced over the elements, in imitation of our Lord’s thanksgiving before breaking the bread. [I must dissent from the opinion that the language is “inexact:” he expresses himself naturally as one who believes it is bread, but yet not “common bread.” So Gelasius, Bishop of Rome (a.d. 490), “By the sacraments we are made partakers of the divine nature, and yet the substance and nature of bread and wine do not cease to be in them,” etc. (See original in Bingham’s Antiquities, book xv. cap. 5. See Chryost., Epist. ad. Cæsarium, tom. iii. p. 753. Ed. Migne.) Those desirous to pursue this inquiry will find the Patristic authorities in Historia Transubstantionis Papalis, etc., Edidit F. Meyrick, Oxford, 1858. The famous tractate of Ratranin (a.d. 840) was published at Oxford, 1838, with the homily of Ælfric (a.d. 960) in a cheap edition.]
To clarify why different groups understand this passage differently per footnote 1911, there is this.
Bettenson states that Justin says:
the food blessed by the prayer of the word which we received from him, by which, through its transformation, our blood and flesh is nourished, this food is the flesh and blood of Jesus who was made flesh.
Bettenson seems to leave the door open for a transformation of the elements.
On the other hand, Thompson translates Justin thus:
the food, “eucharized” by the formula of prayer which comes from Him, and from which our flesh and blood are nourished by transformation, is the flesh and blood of that incarnate Jesus.3
Martyrs of Lyons
Overlapping the end of Justin Martyr's life were the accounts of Christian catechumens charged with various crimes. This account is from Irenaeus.
For when the Greeks, having arrested the slaves of Christian catechumens, then used force against them, in order to learn from them some secret thing [practised] among Christians, these slaves, having nothing to say that would meet the wishes of their tormentors, except that they had heard from their masters that the divine communion was the body and blood of Christ, and imagining that it was actually flesh and blood, gave their inquisitors answer to that effect. Then these latter, assuming such to be the case with regard to the practices of Christians, gave information regarding it to other Greeks, and sought to compel the martyrs Sanctus and Blandina to confess, under the influence of torture, [that the allegation was correct]. To these men Blandina replied very admirably in these words: “How should those persons endure such [accusations], who, for the sake of the practice [of piety], did not avail themselves even of the flesh that was permitted [them to eat]?”
Fragment XIII emphasis mine
In other words, Blandina was saying why would they eat human flesh when they wouldn't even eat animal flesh? They wouldn't. It was false to assert the bread and wine in Thanksgiving (Eucharist) was actual flesh and blood.