Zayd bin Aslam (may Allah have mercy on him) says that al-Qasim (may Allah have mercy on him) was asked about the waiting period of a slave woman, and he said: People say two menstrual cycles, but we do not know anything about that in the Book of Allah or in the Sunnah of the Messenger (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرجعة / 15172