Musannaf Ibn Abi Shayba 26:592

Ibn ‘Ulayyah narrated to us, from Sa‘id, from Qatadah, and I found it in another place from Sa‘id ibn al-Musayyib, that he used to see no harm in selling the *wala’* (patronage rights) if it was from a *mukātabah* (a slave who has entered into a contract of manumission), but he disliked it if it was from manumission.