Musnad Ahmad

Safwan bin Salim says that Hazrat Abu Umamah (may Allah be pleased with him) entered Damascus and saw the heads of the Khawarij hanging. He said three times, "They are the dogs of Hell. Under the shade of the sky, they are the worst of those killed, and under the shade of the sky, the best of those killed was the one whom they martyred." And he began to weep. After a little while, when he returned, someone asked, "O Abu Umamah! This statement of yours, 'dogs of Hell,' did you hear it from the Noble Prophet (peace and blessings be upon him) or are you saying it from your own opinion?" He replied, "Subhan Allah! If I had heard something from the Noble Prophet (peace and blessings be upon him) up to seven times and then omitted the mention of the Noble Prophet (peace and blessings be upon him) in between, I would be very bold indeed." He was asked, "Then why were you weeping?" He replied, "Because these people have gone out of Islam, and these are the very people who created division and split their religion into different groups."