Musannaf Ibn Abi Shayba 13:2697

Abu Bakr narrated to us, he said: Jarir narrated to us, from Layth, he said: Umar ibn al-Khattab said: If a polytheist has a slave and he (the slave) embraces Islam, he is to be taken away from him, and he is to be sold to the Muslims, and his price is to be returned to his owner.