Abu Ghalib said: I was with Sayyiduna Abu Umamah (may Allah be pleased with him) when the heads of the Khawarij were brought and placed on the minarets of Damascus. He said: "Dogs of Hell," saying it three times, and he said: "The worst of the slain under the sky; and the best are those who kill them and those killed by them," saying it three times. I asked: Are you saying this yourself, or did you hear it from the Prophet (peace and blessings be upon him)? He said: Am I so bold? I heard this from the Prophet (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16782