Matthew 19:3-9

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 made them from the beginning made them male and female,
5 and said, 'For this reason a man shall leave his father and mother and be joined 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 put asunder."
7 They said to him, "Why then did Moses command one to give a certificate of divorce, and to put her away?"
8 He said to them, "For 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 unchastity, and marries another, commits adultery."