Abu Ghalib said: The heads of the Kharijites were brought and set up on the steps of Damascus. Sayyiduna Abu Umamah (may Allah be pleased with him) looked at them while they were set up and said: "These are the worst of the slain beneath the sky" - three times - "glad tidings to the one who killed them, and glad tidings to the one whom they killed." I said: O Abu Umamah, do you say this from your own opinion, or is it something you heard from the Messenger of Allah (peace and blessings be upon him)? He said: "I would never be so bold" - three times - "I heard the Messenger of Allah (peace and blessings be upon him) say it."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 8007