Musannaf Ibn Abi Shayba 16:227

Abu Bakr narrated to us, he said: Ibn Fudayl narrated to us, from Husayn, from Mujahid, from Ibn Umar, that he used to lick his three fingers when he ate. And he said: The Messenger of Allah (peace be upon him) said: "Indeed, he does not know in which part of his food the blessing lies."