Musnad Ahmad 32:2095

'Affan narrated to us, Sa'id ibn Yazid narrated to us, 'Ali ibn Zayd narrated to us, he said: I heard Anas ibn Malik saying: "I truly recognize today sins that are finer in your eyes than a hair, which we used to consider among the major sins during the time of the Messenger of Allah, may Allah bless him and grant him peace."