When you see the black banners, go to them even if you have to crawl

It is narrated from Sayyiduna Thawban (may Allah be pleased with him) that he said: When you see that black banners have come out from the direction of Khurasan, then be sure to reach them, even if you have to drag yourselves (on your knees), for among them will be the caliph of Allah, the Mahdi.
This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it.

It is narrated from Sayyiduna Abdullah bin Amr (may Allah be pleased with them both), in the narration of Sayyiduna Salman bin Rabi'ah (may Allah be pleased with him), that he said: I reached Makkah with a group of my companions. We looked for Sayyiduna Abdullah bin Amr but did not find him; there were about three hundred people on foot there. Then we turned back and found him in the mosque. He was an old man wearing two Qatari cloaks and a turban, but he was not wearing a shirt. He asked: Where are you from? We said: From the people of Iraq. He said: You people of Iraq lie, deny the truth and mock. We said: We neither lie, nor deny, nor mock. He asked: How far is it between you and al-Ubullah? We said: Four farsakhs. He said: Soon the Banu Qanturah bin Karkar will drive you out of Khurasan and Sijistan with great harshness. Then they will advance until they tie their horses at the river Tigris. They will be a people with small eyes and flat noses, their faces like layered shields.
This hadith is sahih (authentic) on the condition of Imam Muslim, but the two Shaykhs did not record it.

Zayd bin Sallam narrates from his father, and he from his grandfather, that when the time of death drew near for Sayyiduna Hudhayfah bin al-Yaman (may Allah be pleased with him), some Bedouin people came to him and said: O Hudhayfah! We see that your soul is about to be taken. He said: A joy that came after long waiting, and a beloved who arrived at a time of need. He who regrets (leaving the world) will never succeed. O Allah! I never joined any traitor in his treachery, so today I seek refuge in You from an evil companion. (Then he said:) People used to ask the Messenger of Allah (peace and blessings be upon him) about good, and I used to ask him (peace and blessings be upon him) about evil. I said: O Messenger of Allah! We were in evil, then Allah granted us good; so will there be evil after this good? He (peace and blessings be upon him) said: "Yes." I said: Will there be any evil behind that good too? He (peace and blessings be upon him) said: "Yes." I asked: How? He (peace and blessings be upon him) said: "After me there will be rulers who will neither follow my guidance nor adhere to my Sunnah, and soon there will arise men whose hearts will be the hearts of devils in human bodies." I said: If I encounter this situation, what should I do? He (peace and blessings be upon him) said: "You should listen to and obey the supreme ruler, even if he strikes your back and seizes your wealth."
This hadith has a sahih (authentic) chain of narration, but the two Shaykhs did not record it.

It is narrated from Sayyiduna Abu Mas'ud al-Ansari (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "This matter (of rule) will remain among you, and you will remain its governors, as long as you do not do deeds that strip it away from you. When you do that, Allah will set over you the worst of His creation, who will peel you apart just as the bark of a branch is peeled off."
This hadith has a sahih (authentic) chain of narration, but the two Shaykhs did not record it.

It is narrated from Sa'd bin Hudhayfah (may Allah have mercy on him) that the faults of Sa'id bin al-As were mentioned before Sayyiduna Hudhayfah (may Allah be pleased with him), and he said: I do not know which of the two things you want. Do you want to seize the authority of a people that is not yours, or do you want to turn back this tribulation? Yet it has been sent by Allah; it will roam grazing through the earth until it tramples even its own halter. There is no one to turn it back and no one to stop it, and no one who says "Allah, Allah" will be left without being killed. Then Allah Almighty will send people like the scattered clouds of autumn (qaza'an ka-qaza' al-kharif). He said: Al-qaza' means a patch of thin cloud that is like a shadow when it passes beneath many clouds.
This hadith has a sahih (authentic) chain of narration, but the two Shaykhs did not record it.