Matthew 15:4

“For God said, ‘Honour your father and your mother', and, ‘He who speaks evil of father or mother, let him die the death'.”

He points out that God had declared that a man must honour his father and mother and must not say anything that might result in their harm. Indeed were they to do so they should be subject to capital punishment. Theoretically all his listeners would have agreed with those injunctions. Had He stopped there they would all have solemnly agreed that they believed that as well. But He then points out that in fact they were failing to keep these injunctions because of certain rulings that they had passed, thus invalidating God's word.