Matthew 19:7-9

7They say to him, Why then did Moses command to give a letter of divorce and to send [her] away?

8He says to them, Moses, in view of your hardheartedness, allowed you to put away your wives; but from the beginning it was not thus.

9But I say unto you, that whosoever shall put away his wife, not for fornication, and shall marry another, commits adultery; and he who marries one put away commits adultery.