Musnad Abu Ya'la al-Mawsili 108:167

Abu Bakr narrated to us, Muhammad bin Fudayl narrated to us, from al-A'mash, from Abu Salih and Abu Sufyan, from Jabir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you finishes his food, let him lick his fingers, for he does not know in which part of his food the blessing lies."