Musannaf Ibn Abi Shayba 1:177

Zayd ibn al-Hubab narrated to us, he said: Dawud ibn Abi al-Furat narrated to us, from Muhammad ibn Zayd, from Ibrahim, from al-Aswad ibn Yazid, that Umar ibn al-Khattab performed ablution (wudu), so he inserted his two fingers into the insides of his ears and their outsides, and he wiped them.