Musnad Ahmad 32:872

'Abd al-Rahman narrated to us, Hammad ibn Salamah narrated to us, from Thabit, from Anas, that the Prophet, peace and blessings of Allah be upon him, used to lick his three fingers when he ate. And he said: "If the morsel of one of you falls, let him remove the harm from it and eat it, and let him not leave it for the Shaytan. And let one of you wipe the dish clean, for you do not know in which part of your food the blessing lies."