Sunan Ibn Majah, Chapter: The Book of the Sunnah

"I said to Abu Saeed Khudri: 'Did you hear the Messenger of Allah (saw) mention anything about the Haruriyyah (a sect of Khawarij?' He said: 'I heard him mention a people who would appear to be devoted worshippers: "Such that any one of you would regard his own prayer and fasting as insignificant when compared to theirs. But they will pass through Islam like an arrow passing through its target, then he (the archer) picks up his arrow and looks at its Iron head but does not see anything, then he looks at the shaft and does not see anything then he looks at the band: that which is wrapped around the Iron head where it is connected to the shaft, then he looks at the feather and is not sure whether he sees anything or not." (Sahih)