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