Musannaf Ibn Abi Shayba 9:1195

'Abd al-A'la narrated to us, from Muhammad ibn Ishaq, from al-Zuhri, from Sa'id ibn al-Musayyib, that 'Umar consulted 'Ali ibn Abi Talib (may Allah be pleased with him) and Zayd ibn Thabit. Zayd said, "She has become lawful [to remarry]." 'Ali said, "Four months and ten [days]." Zayd said, "What if she is past the age of childbearing?" 'Ali said, "The later of the two terms." 'Umar said, "If she were to deliver what is in her womb while her husband is on his bier before he has entered his grave, she would have become lawful [to remarry]."