Musannaf Ibn Abi Shayba 18:617

Abu Bakr narrated to us, he said: Ibn Mubarak narrated to us, from Ibn Jurayj, from ‘Ata’, who said: Whatever is gained by the *mukātab* (a slave under a contract of manumission) belongs to him; he uses it to assist himself in his manumission. That is how those who were before you used to say.