you have heard it said (Matt 5:21)

Matthew 5:21

You have heard that it was said....

... But I say to you...

Jesus is not contradicting God's law or changing it, but showing that it goes deeper than they thought. The hebrew language had died out, people were reliant on the communication of God's law to them by the Pharisees. Jesus shows that the communication of external behavioural principles was not at the heart of the law, the law penetrates to the attitudes of the heart. The prophets of the OT would stand up and say "thus says the Lord". Jesus says "I say to you". This is not a man's speculation on God's will but God himself speaking. We should listen.