This is the fundamental presupposition of the whole Sermon on the Mount. Jesus manifests his perfect union with the will of God as revealed in the Old Testament law and prophets. He has in fact nothing to add to the commandments of God, except this, that he keeps them.
Dietrich Bonhoeffer, The Cost of Discipleship (New York: Touchstone, 1995), 122.
God is [the law’s] giver and its Lord, and only in personal communion with God is the law fulfilled. There is no fulfillment of the law apart from communion with God, and no communion with God apart from fulfillment of the law. To forget the first condition was the mistake of the Jews, and to forget the second, the temptation of the disciples.