Musnad Ahmad 33:109

Waki' narrated to us, Sufyan narrated to us, and [in another chain] Abd al-Razzaq, Sufyan informed us, from Abu al-Zubayr, from Jabir, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "When one of you eats food, let him not wipe his hand with the napkin until he licks it—or has it licked—for he does not know in which part of his food the blessing lies."