Musannaf Ibn Abi Shayba 26:578

Ibn ‘Uyaynah narrated to us, from Ibn Abi Najih, from Mujahid, who said: Ali said: "Al-wala’ (the right of inheritance through manumission) is in the status of the alliance (hilf). It is not to be sold, nor is it to be gifted. Leave it as Allah the Exalted has placed it."