Sayyiduna Abu Sa'id (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) concerning the Khawarij that he (peace and blessings be upon him) said: "Their example is that of a man who shot an arrow (at game), then picked it up from where it had fallen and looked at its notch (the rear end of the arrow), and saw no trace of fat or blood on it; then he looked at its head and again saw no trace of fat or blood. So just as nothing of fat or blood clung to that arrow, likewise nothing of Islam has clung to these people."
Hadith Referenceالمعجم الأوسط / باب العين / 4369
Hadith Takhrij«أخرجه الحاكم فى «المستدرك» برقم: 2666»