Deuteronomy - Chapter 24

In the law divorce is permitted:

“Suppose a man marries a woman but she does not please him. Having discovered something wrong with her, he writes a document of divorce, hands it to her, and sends her away from his house."

In the Sermon on the Mount (Matthew 5) Jesus refers to this and says:

“You have heard the law that says, ‘A man can divorce his wife by merely giving her a written notice of divorce.’ But I say that a man who divorces his wife, unless she has been unfaithful, causes her to commit adultery. And anyone who marries a divorced woman also commits adultery."