It is narrated from Abdullah bin Umar (may Allah be pleased with them both) that the waiting period of a free woman is three menstrual cycles, and the waiting period of a slave woman is two menstrual cycles.
Imam al-Bayhaqi (may Allah have mercy on him) said: Someone other than him narrated this hadith from Abdullah bin Umar (may Allah be pleased with them both) as marfu' (attributed to the Prophet), and that is not correct.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15455