Musnad Ahmad 1153:1303

Muhammad ibn Fudayl narrated to us, he said: Al-A’mash narrated to us, from a man, from Masruq, from ‘Aishah, she said: "The right hand of the Messenger of Allah, peace and blessings of Allah be upon him, was for his food and his prayer, and his left hand was for what was other than that."