Musnad Abu Ya'la al-Mawsili 147:26

Ibrahim ibn al-Hajjaj narrated to us, Mahdi ibn Maymun narrated to us, and Abu Khaythama narrated to us, Abd al-Rahman narrated to us, from Mahdi ibn Maymun, from Ghaylan, from Anas, who said: "Indeed, you perform deeds that are finer in your eyes than hair, which we used to consider, during the time of the Messenger of Allah (may Allah bless him and grant him peace), to be the destructive sins (al-mubiqat)."