Musannaf Ibn Abi Shayba 13:190

'Abbad ibn al-'Awwam narrated to us, from Hajjaj, from 'Ikrimah ibn Khalid al-Makhzumi, that a man made a *kitabah* (contract of manumission) with his slave for two slave boys who could perform work like his work. They took the matter up to 'Umar ibn al-Khattab, and he said: "If he does not bring you two slave boys who perform work like his work, then return him to slavery."