Imam al-Shafi'i (may Allah have mercy on him) said: "When the Messenger of Allah (peace and blessings be upon him) affirmed the marriage made in the state of polytheism as valid and kept its people upon it after (accepting) Islam, then, and Allah knows best, nothing is permissible except that the divorce of the people of polytheism also be affirmed as valid."
Sayyidah Aishah (may Allah be pleased with her) said: Marriage in the Jahiliyyah was of four kinds. One of them is the marriage of people today: a man proposes to the guardian of a woman for her, then pays her dowry and marries her.
Imam al-Shafi'i (may Allah have mercy on him) argued from the fact that the Messenger of Allah (peace and blessings be upon him) stoned two Jews who had committed adultery, treating their marriage as what made them muhsan (married and so liable to stoning): so how could we hold that it does not make her lawful when it makes her muhsan?
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14075