Musannaf Ibn Abi Shayba 16:224

Abu Bakr narrated to us, he said: Ibn ‘Uyaynah narrated to us, he said: I said to ‘Ubayd Allah ibn Abi Yazid: "Were you present at the food of Ibn ‘Abbas?" He said: "Yes." I said: "And what did you see him doing?" He said: "I saw him licking his three fingers."