"It has been said, 'Suppose a man divorces his wife. If he does, he must give her a letter of divorce.'(Deuteronomy 24:1)
References for Matthew 5:31
But here is what I tell you. Anyone who divorces his wife causes her to commit adultery. And anyone who gets married to the divorced woman commits adultery. A man may divorce his wife only if she has not been faithful to him.