It is narrated from Sayyiduna Dhu Mikhbar (may Allah be pleased with him) that he said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Soon you will make a secure peace with the Romans; then you and they together will fight a common enemy, and you will gain victory, spoils and safety. Then you will return until you camp in a green meadow with hillocks, and a man from the people of Christianity will raise a cross and say: The cross has prevailed. At this a man from the Muslims will become angry; he will go up to it and break it. At that point the Romans will act treacherously and gather for the great battle."
Hadith Referenceالمعجم الكبير للطبراني / باب الذال / 4215