Is Jesus Quoting The Pharisees Or The Old Law In Matt 5:27?

I am puzzled that some gospel preachers teach Jesus is quoting the false teaching of the Pharisees in Matthew 5:21-48, when in all six cases therein, we can find precisely what Jesus said in the Old Testament law. Another good example is verse 27 where Jesus says "Ye have heard that it was said by them of old time, Thou shalt not commit adultery," an exact quote from the ten commandments in the Old Testament. How could an exact quote of Exod 20:14 be a false interpretation of Old Testament teaching? If you just quote Matt 19:9 with no comment, could an unscripturally married person correctly accuse you of falsely interpreting Jesus’ teaching on MDR?The truth about 27ff is - as in all six cases in Matt 5:21-48 - Jesus quotes an Old Testament verse, and then proceeds to give His stricter New Testament teaching - "whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart."