Musnad Ahmad 434:2

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 Prophet, may God bless him and grant him peace—who said: I heard the Messenger of God, may God bless him and grant him peace, saying: "The Romans will make a secure peace with you, then you will fight alongside them against an enemy. 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, rise up to him, and break it. At that point, the Romans will commit treachery and gather for the great battle."