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."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / Q14075