Humayd bin Mihran said: I asked Abu Ghalib about this verse: "It is He who has sent down to you the Book; in it are verses that are precise — they are the foundation of the Book — and others that are ambiguous," up to "seeking its interpretation" [Al Imran: 7]. He said: Sayyiduna Abu Umamah (may Allah be pleased with him) narrated to me from the Messenger of Allah (peace and blessings be upon him) that he (peace and blessings be upon him) said: "They are the Khawarij." And I asked him about this verse: "As for those whose faces turn black, (it will be said to them): Did you disbelieve after your faith? Then taste the punishment for what you used to disbelieve" [Al Imran: 106]. He said: Sayyiduna Abu Umamah (may Allah be pleased with him) narrated to me from the Messenger of Allah (peace and blessings be upon him) "that they are the Khawarij."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 8015