Musannaf Ibn Abi Shayba 9:1193

Ibn ‘Uyaynah narrated to us, from al-Zuhri, from Salim, who said: I heard a man from the Ansar narrating from Ibn ‘Umar, who said: I heard your father [‘Umar ibn al-Khattab] say: "If the woman whose husband has died were to deliver the child in her womb while he is still upon the bier, she would have become lawful [to remarry]."