Musannaf Ibn Abi Shayba 16:226

Abu Bakr narrated to us, he said: Ibn Fudayl narrated to us, from al-A'mash, from Abu Salih and Abu Sufyan, from Jabir, who said: The Messenger of Allah (may Allah bless him and grant him peace) 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 for him."