The great battles will only take place at the hands of a man from the family of Heraclius

It is narrated from Ka'b (may Allah be pleased with him) that he said: "The great battles (al-malahim) will take place only at the hands of the fourth or fifth man from the family of Heraclius, who will be called 'Tabarah'."

It is narrated from Abu Maryam, the freed slave of Sayyiduna Abu Hurayrah (may Allah be pleased with him), that Sayyiduna Abu Hurayrah (may Allah be pleased with him) passed by Marwan while he was building his house in the middle of Madinah. (Abu Maryam said:) I sat with him while the labourers were working, and Abu Hurayrah (may Allah be pleased with him) said: "Build strong buildings, cherish far-reaching hopes, and die soon." At this Marwan said: "Abu Hurayrah is talking to the labourers. O Abu Hurayrah! What are you saying to them?" He replied: I said: Build strong buildings, cherish far-reaching hopes, and die soon. O people of Quraysh! (He said this three times.) Remember what you were yesterday and how you have become today, that you are being served; the people of Persia and Rome are your slaves; you eat bread of the finest sifted flour (khubz al-samidh) and fat meat. Beware! Do not devour one another (through injustice), nor bite one another as mules bite one another. Be small (through humility) today, and you will be great tomorrow (in the Hereafter). By Allah! Whoever among you raises his own rank (in this world through arrogance), Allah Almighty will lower him on the Day of Resurrection.

It is narrated from Sayyiduna Thawban (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Three men will fight near your treasure, all of them sons of a caliph, then it will not go to any one of them. Then black banners will appear from the direction of the east, and they will fight you in a way that no people have ever fought." Then he (peace and blessings be upon him) mentioned some matters and said: "When you see him, pledge allegiance to him, even if you have to crawl over snow, for he is the caliph of Allah, the Mahdi."
This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim).

It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "In times of trials, the best of people is a man who holds the reins (or he said: the rope) of his horse in pursuit of the enemies of Allah, frightening them while they frighten him; or a man who secludes himself in his desert dwelling, fulfilling the right of Allah that is due upon him."
This hadith is on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it.