Musannaf Ibn Abi Shayba 9:1191

Yazid ibn Harun narrated to us, from Ibn Sa'id, from Sulayman ibn Yasar, from Abu Salamah, who said: I, Ibn Abbas, and Abu Hurayrah were together, and we discussed the man who dies leaving behind a wife, and she gives birth shortly after his death. I said: "When she gives birth, she has become lawful [to remarry]." Ibn Abbas said: "Her waiting period is the longer of the two terms." They debated this matter. Then Abu Hurayrah said: "I am with my nephew," meaning Abu Salamah. So they sent Kurayb, the freed slave of Ibn Abbas, to Umm Salamah. She said: "Subay'ah al-Aslamiyyah gave birth forty nights after the death of her husband. A man from Banu 'Abd al-Dar, whose kunya was Abu al-Sanabil, proposed to her and informed her that she had become lawful [to remarry]. She wanted to marry someone else, but Abu al-Sanabil said to her: 'You have not become lawful.' Subay'ah mentioned that to the Messenger of Allah (peace be upon him), and he commanded her to marry."