Musannaf Ibn Abi Shayba 37:114

Muhammad ibn Abd Allah al-Asadi narrated to us, from Isra'il, from Simak, from Tharwan ibn Milhan, who said: We were sitting in the mosque when Ammar ibn Yasir passed by us. We said to him, "Narrate to us a hadith of the Messenger of Allah, peace and blessings of Allah be upon him, regarding the tribulation (fitna)." He said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "There will be after me leaders who will fight one another for the kingdom; some of them will kill others for it." We said to him, "If anyone other than you had narrated this to us, we would have called him a liar." He said: "As for that, it will surely happen."