In Matthew Chapter 4 during the temptation of Jesus, Jesus uses the words "It is written..." when referring to God's word. In Matthew chapter 5 Jesus begins using the phrase "Ye have heard that it hath been said..." or "Ye hath heard that it was said by them of old time..." when referring to God's word.
Why would Jesus start using a different phrase instead of continuing to say "it is written"?
In the temptation story, Jesus is quoting a scripture passage, introduced by the words "It is written." The focus at that point is Jesus acknowledging the truth and authority of God's word. He is saying in effect, God has spoken and I must submit to that word.
In the Sermon on the Mount, the focus is different. Jesus here is a rabbi teaching his disciples (Matthew 5.2), and the contrast in chapter 5 is between his teaching and the teaching of other Jewish authorities, such as the Pharisees or other rabbis. So when Jesus says "You have heard that it was said", this is not a reference to the OT scripture at all. It's a reference to interpretations of the scriptures.
For example, what does it mean that "You shall not murder"? Jesus' answer in Matthew 5.21-22 is that other teachers have limited the scope of the commandment to the physical act of killing someone. But Jesus' teaching is that the command is broken if and when I get angry with someone. This is presumably because the inward attitude is what leads to the outward action. The focus on the inner heart and spirit is a regular theme of Jesus' teaching.
This distinction between OT command and Jewish interpretations of that command is especially clear in Matthew 5.43-44. Here Jesus quotes teachers who say "Love your neighbour and hate your enemy." In fact this is not a command found in the OT. The only command that is found is to "love your neighbour", so "hate your enemy" is an addition from the earlier Jewish teachers. But in Jesus' view to hate your enemy is to contradict the meaning of the command. For Jesus my "neighbour" is anyone I come in contact with and to whom I can show love. The classic picture of this is the parable of the Good Samaritan.
There is not any meaningful or any significant difference for in both cases the same reality of God's word is referred to. The Holy Scripture was customarily read aloud in synagogues, thus "you have heard" is matching to "it is written".
Thus, Jesus does not refer to Pharisees, Sadducees, or any Jewish theologians' interpretations of the Holy Scriptures when saying "you have heard", but to the Scriptures themselves. The crucial point here is another thing: Jews noticed that Jesus, in difference from Scribes and Pharisees spoke "as a one having authority" (Matthew 7:29). But what does it mean to "have authority"? Why it was so drastically different from the Scribes and Pharisees? Did He speak with a louder voice or with more sparkling eyes displaying a greater inspiration? Not, of course! The reference is not to how rhetorically impressively He spoke but the contents of His speech, the "what" of His speech.
Now, this contents is that He did not take a passage, a quote from Holy Scripture and then interpret it like the Scribes and Pharisees, but changing at will the very passage! Saying that henceforth not the previous passage should be quoted by theologians, but His novel words! That is to say, He substitutes the Scriptural passage with His words, giving a new Scripture. Exactly that is the essence of His scandal, that is the essence of people's bewilderment that "He is speaking as one who has authority to change the very Scripture and establish the new Scripture"! It is the same as when He authoritatively says: "I give you a New Testament" (Matthew 26:28), thus saying that He is either equal to or identical with the one who gave the Old Testament - God. For, in fact, only the one who has the equal and the same authority as God can change at will the words of God.
To give an analogy: if a Roman Emperor legislates: "Roman citizenship should be given only to the inhabitants of Latium and not to anybody else". Then, with changed circumstances in the world, when another Roman Emperor sees that now it will be profitable both for Empire's well-being to give citizenship to others as well, he overrules at will the previous legislation with a new one: "Roman citizenship should be given to all, who will provide a worthy service to it regardless nationality". But only Emperor can overrule an edict of an Emperor. Anybody else, who would do so, would be a criminal. The same here: unless Jesus is God, then overruling the word of God is nothing but a blasphemy. Therefore, the passage gives a clear indication of Jesus' claiming for Himself a divine authority.
One may ask, "but does God oppose God?", for Jesus clearly overrules the Scriptural precepts. Not of course, but God apportions commandments with a consideration of the condition of the listeners' abilities, measuring the commandment as optimal to this condition. In theology this divine consideration, measurement and apportioning is called "economy" (οἰκονομία). Thus, "eye for eye, tooth for tooth" is God's commandment, but the same God made it obsolete after coming to earth in human flesh to teach people the perfect gospel, that they may become as perfect as their Heavenly Father who has sent Him to them.
The purpose of the Sermon on the Mount
If there is one thing the Gospel of Matthew does, it introduces Jesus to NT readers as the new Moses (the New Law Giver), the High Priest who offers a better sacrifice, and the triumphant King in the line of David. In fact, if you follow the Narrative from start to finish, it establishes Jesus as the prophet who escapes the deadly clutches of a ruler who decreed His death, like Moses. He goes through the wilderness in forty days (as opposed to 40 Years), but succeeding after trial unlike the nation of Israel's wanderings. And with Matthew's dozens of fulfillment passages, it's not difficult to see Christ as bringing a special completion to the Old Testament narratives.
So we shouldn't find it strange that Jesus would ascend upon a mountain and explain the requirements for entry into His Kingdom. The parallel to Moses atop Sinai is intentional. We shouldn't make a mistake and regard the Sermon on the Mount as an elaboration of the Law of Moses. Nor should we regard it as something His listeners would have been familiar with. The purpose of the Sermon on the Mount was to establish a law that fulfilled the old Mosiac Covenant. The Law of Christ sounded different because it was different in nature. The Old law was etched on tablets of stone. The New Covenant was promised in Jeremiah to be written on human hearts.
Consider the following:
After Jesus ascended mountain, He explains the blessings of the New Covenant known to readers as the Beatitudes. Later he utters:
21 “You have heard that it was said to the people long ago, ‘You shall not murder,[a] and anyone who murders will be subject to judgment.’ 22 But I tell you that anyone who is angry with a brother or sister[b][c] will be subject to judgment. Again, anyone who says to a brother or sister, ‘Raca,’[d] is answerable to the court. And anyone who says, ‘You fool!’ will be in danger of the fire of hell.
These words and others were entirely new to His listeners. His teachings were targeted to the hearts and minds of the crowd. Change of heart can only come through the new birth. Remember, Jesus did not do away with the law; the Law had eternal ramifications. Jesus fulfilled the law by nature of a more supreme sacrifice.
Jesus prefaced the terms of the New Covenant with "You have heard that it was said", because He is the prophet greater than Moses. Jesus had the authority to establish the Law of Christ.
2 Carry each other’s burdens, and in this way you will fulfill the law of Christ. (Gal 6:2)
