It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that he mentioned the story of Sayyidah Barirah (may Allah be pleased with her). He said that Sayyiduna Abu Bakr (may Allah be pleased with him) said: "The Messenger of Allah (peace and blessings be upon him) set her waiting period as the waiting period of a free woman."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15603