Musannaf Ibn Abi Shayba 13:2699

Abu Bakr narrated to us, he said: Hafs narrated to us, from Ash'ath, from al-Hasan, who said: If the *umm walad* (concubine who has borne a child) of a Christian becomes Muslim, she strives to pay her value. And if a slave woman becomes Muslim, he sells her.