Musnad Ahmad 937:266

Abu ‘Amir narrated to us, ‘Abd al-Jalil—meaning Ibn ‘Atiyyah—narrated to us, Muzahim ibn Mu’awiyah al-Dabbi narrated to us, from Abu Dharr, that the Prophet (peace and blessings of Allah be upon him) went out during the wintertime while the leaves were falling. He took hold of two branches of a tree, and he said: The leaves began to fall. He said: So he said, "O Abu Dharr." I said, "At your service, O Messenger of Allah." He said, "Indeed, the Muslim servant performs the prayer, desiring by it the Face of Allah, so his sins fall away from him just as these leaves fall from this tree."