Musannaf Ibn Abi Shayba 16:237

Abu Bakr narrated to us, he said: 'Abdah narrated to us, from Hisham ibn 'Urwah, from 'Abd al-Rahman ibn Sa'd, from Ibn al-Ka'b, from his father, that the Messenger of Allah (may Allah bless him and grant him peace) used to eat with his three fingers and lick them.