Sayyiduna Abu Umamah (may Allah be pleased with him) saw the heads of the Khawarij and said: "The worst of the slain beneath the shade of the sky." I said: Is this something you say from your own opinion, or something you heard from the Messenger of Allah (peace and blessings be upon him)? He said: "Had I not heard it from the Prophet (peace and blessings be upon him) once, or twice, or three times — up to seven times — I would not have narrated it."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 8013