Musnad Ahmad 31:301

Yazid narrated to us, Muhammad ibn ‘Amr narrated to us, from Abu Salamah, who said: A man came to Abu Sa’id and said: "Did you hear the Messenger of Allah, peace and blessings of Allah be upon him, mention anything regarding the Haruriyyah?" He said: "I heard him mention: 'A people who will delve deeply into the religion. One of you will consider his prayer insignificant compared to their prayer, and his fasting compared to their fasting. They will pass through the religion just as an arrow passes through the game animal.'" "He takes his arrow, and he looks at its blade, and he sees nothing. Then he looks at its shaft, and he sees nothing. Then he looks at its notch, and he sees nothing. Then he looks at its feathers, and he doubts whether he sees anything or not."