Musnad Abu Ya'la al-Mawsili 148:64

Ibrahim ibn al-Hajjaj al-Sami narrated to us, Mahdi ibn Maymun narrated to us, from Ghaylan ibn Jarir, from Anas, who said: "Indeed, you perform deeds that are finer in your eyes than a hair, which we used to consider, during the time of the Messenger of Allah (peace and blessings of Allah be upon him), among the destructive sins."