Musannaf Ibn Abi Shayba 13:2702

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Al-Rabi’ narrated to us, from Al-Hasan, he said: If the slave of a Dhimmi (a non-Muslim subject) accepts Islam, he is brought to the Imam, who sells him to the Muslims and gives his price to his master. And Al-Hasan said: A Muslim is not made the property of a disbeliever.