Musannaf Ibn Abi Shayba 38:153

Yazid ibn Harun narrated to us, he said: Muhammad ibn ‘Amr narrated to us, from Abu Salamah, who said: I said to Abu Sa’id al-Khudri, "Did you hear the Messenger of Allah (may Allah bless him and grant him peace) mention anything regarding the Haruriyyah?" He said, "Yes, I heard him mention a people who exert themselves in worship, such that one of you would consider his own prayer and fasting insignificant compared to their fasting. They pass through the religion just as an arrow passes through the target." "He takes his arrow and looks at its blade, but sees nothing. He looks at its binding, but sees nothing. He looks at its shaft, but sees nothing. He looks at its feathers, and he doubts whether he sees anything or not."