Musnad Ahmad 33:1120

Hassan narrated to us, Ibn Lahi'ah narrated to us, Abu al-Zubayr narrated to us, from Jabir, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you eats and his morsel falls, let him remove what is upon it of harm, then let him eat it, and let him not leave it for the Shaytan. And let not one of you wipe his hand with a cloth until he licks his hand, for the man does not know in which part of his food the blessing is for him. For the Shaytan lies in wait for the son of Adam at every thing, even at his food."