I heard the Messenger of Allah ﷺ say, "The Khawarij are the dogs of Hellfire."
حدثنا إسحاق بن يوسف عن الأعمش
سمعت رسول الله ﷺ يقول الخوارج هم كلاب النار
No references are available.
Its isnad is sahih, and it is found in Ibn Majah 1/61, no. 173, though Busiri said it contains a break.