Musnad Ahmad 32:2145

'Affan narrated to us, Hammad narrated to us, he said: Thabit informed us, from Anas, that the Messenger of Allah (peace and blessings of Allah be upon him) used to lick his three fingers when he ate food. And he said: "If a 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 he commanded us to wipe the dish clean, and he said: "For you do not know in which part of your food the blessing lies."