Ibn Jurayj (may Allah have mercy on him) says: I said to Ata' (may Allah have mercy on him): "If the master asked the slave about his wealth and he concealed it?" He said: "It belongs to the master." Ibn Jurayj (may Allah have mercy on him) says: I asked Ata' (may Allah have mercy on him): "If he concealed a child of his from a slave woman, or he was not asked about it?" He said: "That too belongs to the master." Ibn Jurayj (may Allah have mercy on him) says: "If the master knew of the slave's child but it was not mentioned at the time of the kitabah?" He said: "That too is the master's property."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21706