Musnad Ahmad 919:1

Abdullah narrated to us, Muhammad ibn Abi Bakr al-Muqaddami narrated to us, Abd al-Wahhab al-Thaqafi informed us, from al-Muthanna, from Amr ibn Shu’ayb, from his father, from Abdullah ibn Amr, from Ubayy ibn Ka’b, who said: I said to the Prophet, peace and blessings of Allah be upon him: "‘And for those who are pregnant, their term is until they deliver their burden’ [At-Talaq: 4]—is this for the woman divorced three times, or for the woman whose husband has died?" He said: "It is for the woman divorced three times, and for the woman whose husband has died."