Musnad Ahmad 33:437

Abu Ahmad narrated to us, Sufyan informed us, from Abu al-Zubayr, from Jabir, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "If a morsel of food falls from the hand of one of you, let him remove whatever filth was upon it, and let him not leave it for the Shaytan. And let him not wipe his hand with a napkin until he has licked his fingers, for he does not know in which part of his food the blessing lies."