Mu'awiyah bin Qurrah narrates from his father that the Prophet (peace and blessings be upon him) sent his father, the grandfather of Mu'awiyah, to a man who had consummated marriage with his father's wife; he ordered him, so he struck his neck and took one-fifth of his property.
Our companions said: Striking the neck and taking one-fifth of the property apply only to the apostate, so it seems he had deemed it lawful while knowing it was forbidden. And Allah knows best.
Imam al-Shafi'i (may Allah have mercy on him) said: Sayyiduna Mu'awiyah (may Allah be pleased with him) asked Sayyiduna Ibn Abbas and Sayyiduna Zayd bin Thabit (may Allah be pleased with them both) about the inheritance of the apostate, and they answered: It goes to the public treasury (bayt al-mal). Imam al-Shafi'i (may Allah have mercy on him) said: They meant that it is fay'.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المرتد / 16894