Musnad Ahmad 1153:2266

Muhammad ibn Ja'far narrated to us, from Sa'id, from Abu Ma'shar, from al-Nakha'i, from 'Aishah, she said: "The left hand of the Messenger of Allah, peace and blessings of Allah be upon him, was for his relieving himself and whatever was of filth. And his right hand was for his ablution and for his food."