And he left there and went to the region of Judea and beyond the Jordan, and crowds gathered to him again. And again, as was his custom, he taught them.
And Pharisees came up and in order to test him asked, “Is it lawful for a man to divorce his wife?” He answered them, “What did Moses command you?” They said, “Moses allowed a man to write a certificate of divorce and to send her away.” And Jesus said to them, “Because of your hardness of heart he wrote you this commandment. But from the beginning of creation, ‘God made them male and female.’ ‘Therefore a man shall leave his father and mother and hold fast to his wife, and the two shall become one flesh.’ So they are no longer two but one flesh. What therefore God has joined together, let not man separate.”
And in the house the disciples asked him again about this matter. And he said to them, “Whoever divorces his wife and marries another commits adultery against her, and if she divorces her husband and marries another, she commits adultery.”
(Mark 10:1-12 ESV)