Nafi' narrates from Ibn Umar (may Allah be pleased with them both) that he said: When the master of an umm walad dies, the ruling concerning her waiting period ('iddah) is that she observes a waiting period of one menstrual cycle.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب عتق أمهات الأولاد / 21810
Ibn Sirin (may Allah have mercy on him) said: When a man buys a young slave girl who has not yet reached the age of menstruation, he observes istibra' (waiting to establish that the womb is free) with her for three months.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب عتق أمهات الأولاد / 21811