Musnad Abu Ya'la al-Mawsili 109:247

Abu Khaythama narrated to us, Rawh narrated to us, Ibn Jurayj narrated to us, he said: Abu al-Zubayr informed me that he heard Jabir, that he heard the Messenger of Allah (may Allah bless him and grant him peace) saying: "Let not one of you wipe [his hand] with a napkin until he licks his hand, for he does not know in which part of his food the blessing lies. And indeed, Satan lies in wait for people at every thing, even at their food. And let him not lift the bowl until he licks it, for in the end of the food is the blessing."