Abu Qabil (may Allah have mercy on him) relates that people mentioned the conquest of Constantinople and Rumiyyah (Rome): which of them would be conquered first? So Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) called for a chest, opened it and said: We used to sit with the Messenger of Allah (peace and blessings be upon him) and write down whatever we heard from him (peace and blessings be upon him). We asked: O Messenger of Allah, which of the two cities will be conquered first? He (peace and blessings be upon him) said: "The city of Heraclius." He (peace and blessings be upon him) meant the city of Constantinople.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13763