Ibn Jurayj (may Allah have mercy on him) says: I said to Ata' (may Allah have mercy on him): "A man made a kitabah contract with his slave and settled with him, and the slave concealed from him property of his, slaves and other wealth." He said: "It belongs to his master."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21705