Musannaf Ibn Abi Shayba 9:625

Ibn Numayr narrated to us, from Yahya ibn Sa'id, from Nafi', that Safiyyah bint Abi 'Ubayd had a slave, and she married him to a virgin slave girl. She (the slave girl) used to dislike her husband, and she (Safiyyah) wanted to emancipate her, fearing that if she emancipated the girl, she would be separated from her husband. So, she emancipated the slave first. Then, once the emancipation of the slave was established, she emancipated the girl after that.