It is narrated from Sayyidah Aishah (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) set the waiting period of Barirah as that of a divorced woman when she separated from her husband. Abu Amir al-Aqadi narrated it from Abu Ma'shar and said: The Messenger of Allah (peace and blessings be upon him) ordered her "to observe the waiting period of a free woman." And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15604