Musnad Ahmad 1018:96

Rawh narrated to us, al-Awza’i narrated to us, from Hassan ibn ‘Atiyyah, from Khalid ibn Ma’dan, from Dhu Mikhmar, a man from the companions of the Messenger of Allah, may Allah bless him and grant him peace, who said: I heard the Messenger of Allah, may Allah bless him and grant him peace, say: "The Romans will make a secure peace with you, then you will fight an enemy along with them, and you will be victorious, remain safe, and take spoils. Then you will depart until you encamp in a meadow with mounds. A man from the Christians will raise a cross and say: 'The cross has prevailed.' A man from the Muslims will become angry, stand up to him, and break it. At that point, the Romans will commit treachery and gather for the great battle."