Nafi' (may Allah have mercy on him) narrates from Ibn Umar (may Allah be pleased with them both) that he disliked making a kitabah contract with a slave who knew no trade, and he would say: "You will feed me the filth of people."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21605