John Gill Commentary Matthew 19:7

"They say unto him, Why then did Moses command to give a bill of divorcement, and to put [her] away?" — Matthew 19:7 (ASV)

They say to 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 did not know how to reconcile to the doctrine Christ had delivered, concerning the original institution of marriage, and the close union between a man and his wife by virtue of it, 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)