Musnad Ahmad Hadith 22208

Abu Ghalib says that some heads of the Khawarij were brought from Iraq and hung at the gate of the mosque of Damascus. Abu Umamah (may Allah be pleased with him) came and began to weep, and said three times, "They are the dogs of Hell," and three times, "The worst of those killed under the shade of the sky," and "The best of those killed under the shade of the sky was the one whom they martyred." After a little while, when he returned, someone asked, "Then why were you weeping?" He replied, "I felt pity for them." O Abu Umamah! This statement of yours, 'the dogs of Hell,' did you hear it from the Prophet (ﷺ), or are you saying it from your own opinion?" He replied, "Subhan Allah! If I have heard something from the Prophet (ﷺ) up to seven times, and then omit the mention of the Prophet (ﷺ) in between, then I would be very bold indeed."