Abdullah bin Bishr al-Ghanawi relates that his father, Bishr al-Ghanawi (may Allah be pleased with him), told him that he heard the Messenger of Allah (peace and blessings be upon him) say: "Constantinople will surely be conquered, and what an excellent commander its commander will be, and what an excellent army that army will be." Abdullah said: Then Maslamah bin Abd al-Malik summoned me and asked me about this hadith, so I narrated it to him, and he went out on campaign that very year.
Hadith Referenceالمعجم الكبير للطبراني / باب الباء / 1205