Sunan Ibn Majah, Book 1, Hadith 178 (In-Book) ⓘ
It was narrated that Ibn Awfa said: "The Messenger of Allah said: 'The Khawarij are the dogs of Hell.'"
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا إِسْحَاقُ الأَزْرَقُ، عَنِ الأَعْمَشِ، عَنِ ابْنِ أَبِي أَوْفَى، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ " الْخَوَارِجُ كِلاَبُ النَّارِ " .
also referenced under: Hadith 173 (Reference) & Volume 1, Book 1, Hadith 173 (DarusSalam)