Musannaf Ibn Abi Shayba 13:514

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Ibn Abi ‘Arubah, from Qatadah, from Sa’id ibn al-Musayyib, who said: 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). And he used to dislike it if it was from manumission (by direct emancipation).