Musannaf Ibn Abi Shayba 13:183

Hushaym ibn Bashir narrated to us, from Abd al-Hamid, from Siwar, who said: A female relative by marriage of mine, who was called Sarah, a freedwoman of Abu Barzah, narrated to me that Abu Barzah made a contract of manumission (kitabah) with some of his slaves for a price.