Yazid bin al-Bara narrates from his father (may Allah be pleased with him) that he said: I met my uncle carrying a banner, and I said: Where are you headed? He said: The Messenger of Allah (peace and blessings be upon him) has sent me to a man who has married his father's wife, "(and ordered me) to strike his neck and take his wealth."
Some of our companions have taken this to mean that he married her believing it to be permissible, and thereby became an apostate, so that killing him and taking his wealth became obligatory.
Imam al-Shafi'i (may Allah have mercy on him) said: It has been reported that Mu'awiyah (may Allah be pleased with him) wrote to Ibn Abbas (may Allah be pleased with them both) and Zayd bin Thabit (may Allah be pleased with him) asking them about the inheritance of an apostate, and they said: "It goes to the public treasury (bayt al-mal)." Imam al-Shafi'i (may Allah have mercy on him) said: The two of them meant that it is fay'.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12459