Musnad Ahmad 962:177

Anas ibn Iyad narrated to us, saying: I heard Safwan ibn Sulaym saying: Abu Umamah al-Bahili entered Damascus and saw the heads of the Haruriyyah (Kharijites) set up. He said: "The dogs of Hellfire, the dogs of Hellfire," three times. "The worst of the slain under the canopy of the sky, and the best of the slain are those whom they killed." Then he wept. A man stood up to him and said: "O Abu Umamah, is this which you say from your own opinion, or did you hear it?" He said: "I would be very bold indeed if I were to say this from my own opinion. I have heard it not once, nor twice." The man said: "Then what makes you weep?" He said: "I weep because of their exit from Islam—those who divided and took their religion into sects."