Musnad Ahmad 30:2246

'Affan narrated to us, Wuhayb narrated to us, Hisham—meaning Ibn 'Urwah—narrated to us, from a man, from Abu Hurayrah, that the Prophet (peace and blessings of Allah be upon him) said: "When one of you eats, let him lick his fingers, for he does not know in which part of it the blessing lies."