Matthew 19:4-8

And he answered and said unto them, Have ye not read, that he which made them at the beginning made them male and female,

And said, For this cause shall a man leave father and mother, and shall cleave to his wife: and they twain shall be one flesh?

Wherefore they are no more twain, but one flesh. What therefore God hath joined together, let not man put asunder.

They say unto him, Why did Moses then command to give a writing of divorcement, and to put her away?

He saith unto them, Moses because of the hardness of your hearts suffered you to put away your wives: but from the beginning it was not so.