Musnad Abu Ya'la al-Mawsili 109:284

Ibn Numayr narrated to us, Ya‘la narrated to us, from al-A‘mash, from Abu Sufyan, from Jabir, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "When one of you eats, let him lick his fingers, for he does not know in which part of his food the blessing lies."