Musnad Abu Ya'la al-Mawsili 110:177

Muhammad ibn 'Abbad al-Makki narrated to us, Sufyan ibn 'Uyaynah narrated to us, from 'Amr, from 'Ata', from Ibn 'Abbas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "When one of you eats, let him not wipe his hand until he licks it or has it licked."