Ibn Hibban

Sayyiduna Dhi Mukhbir (may Allah be pleased with him), who was the nephew of Najashi, narrates: He heard the Prophet Muhammad (peace and blessings be upon him) saying: "You will make a peace treaty with the Romans, a treaty of security, until you and they together fight against an enemy who is other than them, and you will be helped. You will gain spoils of war, then you will return until you camp in a pasture with hills. Then a man from among the Romans will say: 'The cross has prevailed,' and a man from among the Muslims will say: 'Rather, Allah has granted victory.' Then that Muslim will go towards their cross, which will not be far from him, and he will break it. The Romans will then advance towards the man who broke their cross and will behead him. The Muslims will then go towards their weapons and fighting will begin, and Allah will honor a group of Muslims with martyrdom. The Romans will say to their king: 'We are sufficient to face the Arabs,' then they will gather for a fierce battle and will come towards you under eighty banners, under each banner there will be twelve thousand men."