1 Faustus said: You ask why we do not receive the law and the prophets, when Christ said that he came not to destroy them, but to fulfill them. Where do we learn that Jesus said this? From Matthew, who declares that he said it on the mount. In whose presence was it said? In the presence of Peter, Andrew, James, and John— only these four; for the rest, including Matthew himself, were not yet chosen. Is it not the case that one of these four— John, namely— wrote a Gospel?
It is. Does he mention this saying of Jesus? No. How, then, does it happen that what is not recorded by John, who was on the mount, is recorded by Matthew, who became a follower of Christ long after He came down from the mount? In the first place, then, we must doubt whether Jesus ever said these words, since the proper witness is silent on the matter, and we have only the authority of a less trustworthy witness. But, besides this, we shall find that it is not Matthew that has imposed upon us, but some one else under his name, as is evident from the indirect style of the narrative.
Thus we read: "As Jesus passed by, He saw a man, named Matthew, sitting at the receipt of custom, and called him; and he immediately rose up, and followed Him." No one writing of himself would say, He saw a man, and called him; and he followed Him; but, He saw me, and called me, and I followed Him. Evidently this was written not by Matthew himself, but by some one else under his name. Since, then, the passage already quoted would not be true even if it had been written by Matthew, since he was not present when Jesus spoke on the mount; much more is its falsehood evident from the fact that the writer was not Matthew himself, but some one borrowing the names both of Jesus and of Matthew.
Source: Reply to Faustus the Manichaean (New Advent)