Overview
The Prophet ﷺ foretold a future conquest of Constantinople (Istanbul) that will occur through takbeer alone — the Muslims will say "La ilaha illallah" and "Allahu Akbar" and the walls will crumble by the will of Allah. This is distinct from the historical Ottoman conquest of 1453 CE, as the hadith describes specific conditions that were not met in that conquest.
Primary Hadith
Scholarly Context
The hadith in Sahih Muslim describes this conquest occurring after the great battle (Al-Malhama al-Kubra) and immediately before the emergence of the Dajjal. While the Muslims are distributing the spoils of Constantinople, a false cry will be raised that the Dajjal has emerged, causing them to rush back. Scholars like Ibn Katheer and As-Suyuti affirmed that the prophesied conquest by takbeer is a future event.