Musnad Ahmad 33:275

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Abu Sufyan, from Jabir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you eats, let him not wipe his hand until he has licked it, for he does not know in which part of his food the blessing lies."