Musannaf Ibn Abi Shayba 1:172

172. Abdullah ibn Idris narrated to us, from Muhammad ibn Ajlan, from Zayd ibn Aslam, from Ata ibn Yasar, from Ibn Abbas, that the Messenger of Allah (may Allah bless him and grant him peace) wiped the inside of his ears with his index fingers. He placed his thumbs behind the outer part of his ears, and thus wiped their insides and their outsides.