It is narrated from Yazid bin Abi Habib (may Allah have mercy on him) that Ibn Shihab (may Allah have mercy on him) wrote to him that Ubaydullah bin Abdullah bin Utbah (may Allah have mercy on him) informed him from his father that he wrote to Ibn al-Arqam (may Allah be pleased with him): Ask Subay'ah al-Aslamiyyah (may Allah be pleased with her) how the Messenger of Allah (peace and blessings be upon him) gave her a ruling when her husband died. She said: "He gave me the ruling that when I had delivered my child, I could marry."
Imam al-Bukhari (may Allah have mercy on him) narrated it in his Sahih from Yahya bin Bukayr.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15469