Musnad Ahmad 962:47

'Abd al-Razzaq narrated to us, Ma'mar informed us, he said: I heard Abu Ghalib saying: When the heads of the Azariqah were brought and set up on the steps of Damascus, Abu Umamah came. When he saw them, his eyes welled up with tears. He said: "Dogs of the Fire," three times. "These are the worst of the slain who have been killed under the canopy of the sky, and the best of the slain who have been killed under the canopy of the sky are those whom these [people] killed." He said: So I said: What is the matter with you that your eyes welled up with tears? He said: Mercy for them, for they were of the people of Islam. He said: We said: Is it by your own opinion that you said "These are the dogs of the Fire," or something you heard from the Messenger of Allah, may Allah bless him and grant him peace? He said: I would be very bold [to say that on my own]. Rather, I heard it from the Messenger of Allah, may Allah bless him and grant him peace, not once, nor twice, nor three times. He said: And he counted many times.