Musannaf Ibn Abi Shayba 13:821

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from al-Jurayri, from Abu ‘Abd Allah al-Jasri—of [the tribe of] ‘Anazah—he said: I said to Ma‘qil ibn Yasar: A man among us wants to free a freed slave (mu‘taq). He said: If you buy a freed slave whom you intend to free, do not stipulate the freeing for his family, for it is a bond of slavery. Rather, buy him silently; if you wish, you may keep him, and if you wish, you may free him.