Ibn Jurayj (may Allah have mercy on him) said to Ata' (may Allah have mercy on him): "The mukatab does not stipulate that whatever children are born will be within my kitabah, and then children are born?" He said: "They are within his kitabah." Amr bin Dinar said the same.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21702