Musnad Ahmad 595:16

Hassan ibn Musa narrated to us, Ibn Lahi'ah narrated to us, al-Harith ibn Yazid narrated to us, from Abd al-Rahman ibn Jubayr, that al-Mustawrid said: While I was with Amr ibn al-Aas, I said to him: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "The most severe of people against you are the Romans, and their destruction will only be at the Hour." Amr said to him: Did I not forbid you from such as this?