Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) reports this saying of the Prophet (peace and blessings be upon him): "When two freed men become rulers, one a freed man of the Arabs and the other a freed man of the Romans, then great battles will take place at the hands of the two of them."
Al-Tabarani recorded it. Its chain contains Ibn Lahi'ah, in whom there is weakness, and the one who narrates from him is Muhammad bin Sufyan; I do not know him.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الفتن - أعاذنا الله منها / 12415