Jesus is referring to Deuteronomy 24:1–4. Reflecting on this passage, the scribes and Pharisees focused on the exception (when is divorce permissible?), whereas Jesus wants people to focus on the principle (what is God’s purpose for marriage). 1 He will give more details on the principle when he responds to a question from the Pharisees in Matthew 19:1–30.
31 “It was also said, ‘Whoever divorces his wife, let him give her a certificate of divorce.’