Musnad Ahmad 928:28

'Abdullah narrated to us, 'Ubaydullah ibn 'Umar narrated to me, Yazid ibn Zuray' and 'Abd al-A'la narrated to us, they said: Dawud narrated to us, from Muhammad ibn Abi Musa, from Ziyad al-Ansari, who said: I said to Ubayy ibn Ka'b: "If all the wives of the Prophet, may Allah bless him and grant him peace, were to die, would it be permissible for him to marry?" He said: "And what would make that forbidden for him?" I said: "Because of His saying: 'It is not lawful for you to take [other] women after [this]' [Al-Ahzab: 52]." He said: "It was only that a certain type of woman was made lawful for the Messenger of Allah, may Allah bless him and grant him peace."