Musannaf Ibn Abi Shayba 16:219

Abu Bakr narrated to us, he said: Suwayd ibn ‘Amr narrated to us, he said: Hammad ibn Salamah narrated to us, from Thabit, from Anas, who said: When the Messenger of Allah (may Allah bless him and grant him peace) ate, he would lick his three fingers. And he said: The Messenger of Allah (may Allah bless him and grant him peace) commanded us to wipe the dish clean. And he said: Indeed, one of you does not know in which part of his food the blessing lies.