Musnad Ahmad 33:821

Abu Nu'aym narrated to us, Sufyan narrated to us, from Abu al-Zubayr, from Jabir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If a morsel of food falls from one of you, let him remove whatever filth has touched it, and let him not leave it for the Shaytan. And let him not wipe his hand with a cloth until he licks it—or has it licked—for he does not know in which part of his food the blessing lies."