Musannaf Ibn Abi Shayba 13:130

Kathir ibn Hisham narrated to us, from Ja'far, from al-Zuhri, who said: The Sunnah has passed that whoever comes to own anything of his prohibited kin (mahram), he becomes free by his ownership, emancipated. He said: As for what is beyond that of kinship, it is a bond of kinship that Allah has commanded to be maintained and has forbidden its severance. And I do not know of anything from the severance of kinship more severe than a man taking his relative as a slave.