Abu Ghalib narrated that Abu Umamah said:

"(The Khawarij) are the worst of the slain who are killed under heaven, and the best of the slain are those who were killed by them. Those (Khawarij) are the dogs of Hell. Those people were Muslims but they became disbelievers." I said: "O Abu Umamah, is that your opinion?" He said: "Rather I heard it from the Messenger of Allah."

حَدَّثَنَا سَهْلُ بْنُ أَبِي سَهْلٍ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنْ أَبِي غَالِبٍ، عَنْ أَبِي أُمَامَةَ، يَقُولُ شَرُّ قَتْلَى قُتِلُوا تَحْتَ أَدِيمِ السَّمَاءِ وَخَيْرُ قَتْلَى مَنْ قَتَلُوا كِلاَبُ أَهْلِ النَّارِ قَدْ كَانَ هَؤُلاَءِ مُسْلِمِينَ فَصَارُوا كُفَّارًا ‏.‏ قُلْتُ يَا أَبَا أُمَامَةَ هَذَا شَىْءٌ تَقُولُهُ قَالَ بَلْ سَمِعْتُهُ مِنْ رَسُولِ اللَّهِ ـ صلى الله عليه وسلم ـ ‏.‏