Imam al-Bayhaqi (may Allah have mercy on him) said: Abu Bakr (may Allah have mercy on him) narrated to us from Ibn Ulayyah (may Allah have mercy on him), who narrates from Layth (may Allah have mercy on him), who narrates from Tawus (may Allah have mercy on him) and Ata' (may Allah have mercy on him), that if (the slave woman) does not menstruate, then her 'iddah is three months.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15594