Musannaf Ibn Abi Shayba 13:193

Waki’ narrated to us, he said: Hammad bin Zayd narrated to us, from Abdullah bin Abi Bakr bin Anas, he said: This is the *mukātabah* (contract of manumission) of Sirin that we have: This is what Anas bin Malik contracted with his slave upon. He contracted with him for such and such amount of thousands, and for two slaves who work like his work.