It is narrated from Sayyiduna Amr bin Awf bin Milhah al-Muzani (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said to Sayyiduna Ali bin Abi Talib (may Allah be pleased with him): "You will fight the Banu al-Asfar (the Romans) along with a band of the Muslims, not caring for the blame of any blamer in the matter of Allah, until you conquer Constantinople through takbir and tasbih."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14100