Mark 1:22

And they were astonished at his teaching: for he taught them as having authority, and not as the scribes.

Matthew reported the close of the sermon on the mount in almost these exact words. Why? Both Matthew, Mark, and all the gospel writers drew freely upon the established oral tradition which existed for about a decade before Matthew wrote and about three decades before Mark wrote. Such an expression as this verse had been repeated perhaps millions of times by believers in recounting the wonderful story of Jesus; and there can be no marvel at all that it is found in both gospels, with just enough variation to show that both authors wrote independently, Matthew writing, "their scribes," and Mark writing "the scribes." It is obvious that nobody copied anybody!

Having authority ... Jesus' teaching was promulgated by him as being superior to that of Moses. He took up the great precepts of the law, repeated them, and then added, "But I say unto you," going on to indicate his own teachings as superior to those of the law (Matthew 5:32,34,39,44 etc.).