Musannaf Ibn Abi Shayba 16:217

Abu Bakr narrated to us, he said: Abu Mu'awiyah narrated to us, from al-A'mash, 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 licks it, for he does not know in which part of his food the blessing lies."