Musnad Ahmad 695:11

Rawh narrated to us, he said: Malik ibn Anas narrated to us, from Hisham ibn Urwah, from his father, that al-Miswar ibn Makhramah informed him, he said: And Ishaq—meaning Ibn al-Taba'—narrated to us, he said: Malik informed me, from Hisham, from his father, from al-Miswar ibn Makhramah, that Subay'ah al-Aslamiyyah gave birth a few nights after the death of her husband. The Messenger of Allah, peace and blessings of Allah be upon him, said to her: "You have become lawful [to remarry], so marry."