It is narrated from Ibn Shihab (may Allah have mercy on him) that Ubaydullah (may Allah have mercy on him) narrated to him that his father, Abdullah bin Utbah (may Allah be pleased with him), wrote to Umar bin Abdullah bin al-Arqam al-Zuhri (may Allah have mercy on him), instructing him to go to Subay'ah al-Aslamiyyah (may Allah be pleased with her) and ask her about her account and about what the Messenger of Allah (peace and blessings be upon him) said to her when she asked him (peace and blessings be upon him) for a ruling. Umar bin Abdullah (may Allah have mercy on him) wrote to Abdullah bin Utbah (may Allah be pleased with him), informing him that Subay'ah al-Aslamiyyah (may Allah be pleased with her) had told him that she had been married to Sa'd bin Khawlah (may Allah be pleased with him), who was of the tribe of Banu Amir bin Lu'ayy and was among those who were present at Badr. He died during the Farewell Hajj while she was pregnant. Not long after his death she delivered her child. When she was free of her postnatal bleeding, she adorned herself for those seeking marriage. Abu al-Sanabil bin Ba'kak (may Allah be pleased with him), a man of Banu Abd al-Dar, came in to her and said to her: "What is the matter with you? I see that you have adorned yourself; perhaps you intend to marry. By Allah, you cannot marry until four months and ten days have passed over you." Subay'ah (may Allah be pleased with her) said: When he said that to me, I gathered my clothes, and when evening came I went to the Messenger of Allah (peace and blessings be upon him) and asked him (peace and blessings be upon him), and he (peace and blessings be upon him) gave me the ruling "that I had become lawful (for marriage) when I delivered my child," and he (peace and blessings be upon him) instructed me to marry if I saw fit.
Ibn Shihab (may Allah have mercy on him) said: I see no harm in her marrying once she has given birth, even if she is still bleeding, except that her husband should not approach her until she becomes pure.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15468