Matthew 19:3-9 (ESV)

3 And Pharisees came up to him and tested him by asking, “Is it lawful to divorce one’s wife for any cause?”

4 He answered, “Have you not read that he who created them from the beginning made them male and female,  5 and said, ‘Therefore a man shall leave his father and his mother and hold fast to his wife, and the two shall become one flesh’?  6 So they are no longer two but one flesh. What therefore God has joined together, let not man separate.”

7 They said to him, “Why then did Moses command one to give a certificate of divorce and to send her away?” 8 He said to them, “Because of your hardness of heart Moses allowed you to divorce your wives, but from the beginning it was not so. 9 And I say to you: whoever divorces his wife, except for sexual immorality, and marries another, commits adultery.”