Musnad Ahmad 434:3

Muhammad ibn Mus’ab—who is al-Qarqasani—narrated to us, he said: al-Awza’i narrated to us, from Hassan ibn ‘Atiyyah, from Khalid ibn Ma’dan, from Jubayr ibn Nufayr, from Dhu Mikhmar, from the Prophet (peace and blessings of Allah be upon him), who said: "You will make a secure peace with the Romans, and you and they will fight an enemy behind them. You will be safe and take spoils. Then you will descend upon a meadow with mounds. A man from the Romans will stand up, raise the cross, and say: 'The cross has prevailed.' A man from the Muslims will stand up to him and kill him. At that, the Romans will commit treachery, and the great battles will occur. They will gather against you and come to you under eighty banners, with each banner being ten thousand."