It is narrated from Ibn Umar (may Allah be pleased with him) that the 'iddah of an umm walad when her master dies, or of a slave woman when she is set free or given away as a gift, that is, presented to someone, is one menstrual period.
It was narrated to us from Abdullah bin Mas'ud (may Allah be pleased with him) that a slave woman is to undergo istibra' with one menstrual period.
It was narrated to us from al-Hasan, Ata', Ibn Sirin and Ikrimah (may Allah have mercy on them) that they all said: She is to undergo istibra' even if she is a virgin.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15592