Musannaf Ibn Abi Shayba 13:2245

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Sufyan narrated to us, from Abd al-Karim, from Nafi’, from Ibn Umar, that he disliked for a man to enter into a *kitabah* (contract of manumission) with his slave if he did not have a trade.