Sayyiduna Bishr al-Khath'ami (may Allah be pleased with him) narrates that he heard the Prophet (peace and blessings be upon him) say: "Constantinople will soon be conquered; its commander will be an excellent commander, and that army will be the best army." The narrator says: Maslamah bin Abd al-Malik called for me, and I narrated this hadith to him, and then he campaigned against Constantinople.
Ahmad, al-Bazzar and al-Tabarani recorded it, and its narrators are thiqat (trustworthy).