Musnad Ahmad 28:169

Yahya ibn Ishaq narrated to us, Yahya ibn Ayyub narrated to us, Abu Qubayl narrated to me, he said: We were with Abdullah ibn Amr ibn al-As, and he was asked: "Which of the two cities will be conquered first: Constantinople or Rome?" Abdullah called for a chest of his that had rings. He said: So he took out a book from it. Abdullah said: "While we were around the Messenger of Allah (may Allah bless him and grant him peace) writing, the Messenger of Allah (may Allah bless him and grant him peace) was asked: 'Which of the two cities will be conquered first: Constantinople or Rome?'" The Messenger of Allah (may Allah bless him and grant him peace) said: "The city of Heraclius will be conquered first," meaning Constantinople.