Musnad Ahmad 175:1

Waki’ narrated to us, he said: Hisham bin ‘Urwah narrated to us, from ‘Abd al-Rahman bin Sa’d, from Ibn Ka’b bin Malik al-Ansari, from his father; and Ibn Numayr, from Hisham, from ‘Abd al-Rahman bin Sa’d, from Ibn Ka’b bin Malik, from his father: "That the Prophet, may Allah bless him and grant him peace, ate food, then he licked his fingers."