Musnad Ahmad 1153:2264

'Abd al-Wahhab narrated to us, from Sa'id, from Abu Ma'shar, from al-Nakha'i, from al-Aswad, from 'Aishah, that she said: The right hand of the Messenger of Allah, peace and blessings of Allah be upon him, was for his purification and his food. And the left hand was for his relieving himself and whatever was of filth.