Abu Ghalib said: Seventy heads of the Azariqah (Khawarij) were brought and kept on the steps of Damascus for seven days. Three days passed, and on the fourth day Sayyiduna Abu Umamah (may Allah be pleased with him) came and prayed two rak'ahs beside a pillar, then said: "The dogs of Hell" — three times — "the worst of the slain beneath the shade of the sky; the best of the slain beneath the shade of the sky are those whom they killed." I said: O Abu Umamah, is this something you heard from the Messenger of Allah (peace and blessings be upon him), or do you say it from yourself? He said: "I could never be so bold. Rather, I heard it from the Messenger of Allah (peace and blessings be upon him) not once or twice, but up to seven times."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 8024