I heard the Messenger of Allah ﷺ say, "The Romans will make a secure peace treaty with you. Then you will campaign together against an enemy, and you will be granted victory, safety, and spoils. Afterwards you will withdraw and camp in a meadow with hillocks, where a man from among the Christians will raise a cross and say, 'The cross has prevailed.' A man from the Muslims will become enraged, rise up against him, and crush it. At that moment the Romans will betray the treaty and gather for the great battle."
حدثنا روح حدثنا الأوزاعي عن حسان بن عطية عن خالد بن معدان
سمعت رسول الله ﷺ يقول ستصالحكم الروم صلحا آمنا ثم تغزون وهم عدوا فتنصرون وتسلمون وتغنمون ثم تنصرون الروم حتى تنزلوا بمرج ذي تلول فيرفع رجل من النصرانية صليبا فيقول غلب الصليب فيغضب رجل من المسلمين فيقوم إليه فيدقه فعند ذلك يغدر الروم ويجمعون للملحمة
Its isnad is sahih, its narrators are Levantines, trustworthy, well-known jurists, except for the sheikh of Ahmad.The hadith was narrated by Abu Dawud, 4/109, no. 4292, in the chapter on what is mentioned regarding tribulations, and Ibn Majah, 2/1369, no. 4089, and he said in Zawaid, "Its isnad is hasan."As for his statement, "then you will support the Romans," it means you will fight alongside them, as mentioned before.