Its not mentioned in the Quran and as far as the Arabs and other Middle Eastern people were concerned, Byzantium was Rome. The people of the Byzantine Empire and Anatolia were not known as Greeks or Byzantine, but rather as Rum (Rumi). When Mehmed declared himself as Kaiser of Rome (i.e. Roman Caesar), he did so with the implication that he was now the new Roman Emperor.

Can you give the detail about where was it mentioned in Quran? I am a muslim and I never heard it before.