Abu Ghalib reported: Abu Umamah, may Allah be pleased with him, said, “The khawarij are the worst of those slain under the heavens, and the best of those slain are those who were killed by them. They are the dogs of Hellfire. They were Muslims and then became unbelievers.” I said, “O Abu Umamah, is this something you say?” Abu Umamah said, “No, rather I heard it from the Messenger of Allah, peace and blessings be upon him.”
Source: Sunan Ibn Mājah 176
Grade: Hasan (fair) according to Al-Albani
عَنْ أَبِي غَالِبٍ عَنْ أَبِي أُمَامَةَ رضي الله عنه قال (الخوارج) شَرُّ قَتْلَى قُتِلُوا تَحْتَ أَدِيمِ السَّمَاءِ وَخَيْرُ قَتِيلٍ مَنْ قَتَلُوا كِلَابُ أَهْلِ النَّارِ قَدْ كَانَ هَؤُلَاءِ مُسْلِمِينَ فَصَارُوا كُفَّارًا قُلْتُ يَا أَبَا أُمَامَةَ هَذَا شَيْءٌ تَقُولُهُ قَالَ بَلْ سَمِعْتُهُ مِنْ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ
176 سنن ابن ماجه كتاب المقدمة باب في ذكر الخوارج
146 المحدث الألباني خلاصة حكم المحدث حسن في صحيح ابن ماجه