Khilas bin Amr reports from Sayyiduna Ali (may Allah be pleased with him) that the 'iddah of an umm walad is four months and ten days.
Imam Waki' (may Allah have mercy on him) said: Its meaning is: when her husband also dies after her master's death.
Imam al-Bayhaqi (may Allah have mercy on him) said: The narrations of Khilas from Sayyiduna Ali (may Allah be pleased with him) are not strong according to the scholars of hadith; they say: it is very weak.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15586