Abu Hurairah reported,
The Messenger of Allah ﷺ said, "The Hour will not come until two great factions fight one another, with a massive slaughter between them, though their claim is one and the same."
Ahmad 10864
Sahihحدثنا علي أخبرنا ورقاء عن أبي الزناد عن الأعرج
عن أبي هريرة قال
قال رسول الله ﷺ لا تقوم الساعة حتى يقتتل فئتان عظيمتان تكون بينهما مقتلة عظيمة ودعواهما واحدة
Its chain of transmission is sahih, narrated by the two Shaykhs in the chapters on tribulations, Bukhari 4/243, and Muslim 4/2214 number 157 repeated. The hadith has been narrated with multiple wordings that together indicate this was information given by the Messenger of Allah ﷺ about what would happen between Ali and Muawiya 
, as some of the narrations specify, "from my nation." It has also been said that this concerns non-Muslims, though I did not find a narration that specifies this, but the general wording may suggest it. We believe in what the Messenger of Allah ﷺ said.