Musannaf Ibn Abi Shayba 13:1388

Abu Bakr narrated to us, he said: Ibn Abi Za’idah narrated to us, from ‘Abd al-Malik, from ‘Ata’, regarding the saying of the Exalted: "And give them from the wealth of Allah which He has given you," he said: "From what Allah has brought out for you from his (the slave's) mukatabah (contract of manumission)."