“Why,” said La Belle Isode,”are ye a knight and are no lover? For sooth, it is a great shame to you; wherefore ye may not be called a good knight by reason but if ye make a quarrel for a lady.”

Thomas Malory (c. 1415-1471) English writer

Le Morte d’Arthur, Book 10, ch. 56 (1485)
    (Source)

Often paraphrased, "The very purpose of a knight is to fight on behalf of a lady."