Matthew 19:7

They say unto him
That is the Pharisees, who object the law of Moses to him, hoping hereby to ensnare him, and expose him to the resentment of the people, should he reject that, as they supposed he would;

why did Moses then command to give a writing of divorcement, and
put her away?
referring to ( Deuteronomy 24:1 ) which they thought to be a contradiction, and what they knew not how to reconcile to the doctrine Christ had delivered, concerning the original institution of marriage, and the close union there is between a man and his wife, by virtue of it, and which is not to be dissolved by men. Concerning a writing of divorcement and the form, and manner of it, (See Gill on Matthew 5:31)