Musannaf Ibn Abi Shayba 13:1582

Abu Bakr narrated to us, he said: Jarir narrated to us, from Mughira, from Ibrahim, who said: It is to be looked at: whatever the people helped him with in his *kitaba* (contract of manumission), he places it towards the freeing of slaves. And whatever was from his own earnings and his own wealth, it belongs to his master.