Musnad Ahmad 962:15

Abu Sa’id narrated to us, ‘Abdullah ibn Bujayr narrated to us, Sayyar narrated to us, he said: Heads were brought from the direction of Iraq and were set up at the gate of the mosque. Abu Umamah came and entered the mosque, then he performed two rak’ahs, then he went out to them. He looked at them and raised his head, and said: "The worst of the slain under the shadow of the sky," three times, "and the best of the slain under the shadow of the sky is the one whom they killed." And he said: "Dogs of the Fire," three times. Then he wept, then he turned away from them. A person said to him: "O Abu Umamah, have you seen this hadith? Where you said: 'Dogs of the Fire,' is it something you heard from the Messenger of Allah, may Allah bless him and grant him peace, or something you say by your own opinion?" He said: "Subhan Allah! I would be very bold indeed. If I had heard it from the Messenger of Allah, may Allah bless him and grant him peace, once or twice—until he mentioned seven times—I would have thought that I would not mention it." The man said: "For what reason did you weep?" He said: "Out of mercy for them, or because of my mercy for them."