Musnad Ahmad 32:592

'Abd al-Samad narrated to us, my father narrated to me, Sinan narrated to us, Anas narrated that the Messenger of Allah (peace and blessings of Allah be upon him) took a branch and shook it, but it did not shed [its leaves]. Then he shook it, and it did not shed. Then he shook it, and it shed. The Messenger of Allah (peace and blessings of Allah be upon him) then said: "Indeed, 'Subhan Allah' (Glory be to Allah), 'Alhamdulillah' (Praise be to Allah), 'La ilaha illa Allah' (There is no god but Allah), and 'Allahu Akbar' (Allah is the Greatest) shed sins just as a tree sheds its leaves."