Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.032B.

The sunnah is to wipe the interior of the ears with the index fingers and the exterior portions with the thumbs. The water used to wipe the head is also used for the ears, as the ears are part of the head. Al-Miqdam ibn Ma’d Yakrih reported that the Prophet, upon whom be peace, wiped his head and his ears, the interior and exterior, while making ablution. He also put his finger inside his ear. (Related by Abu Dawud and at-Tahawi.)

While describing the ablution of the Prophet, upon whom be peace, Ibn ‘Umar said, “He wiped his head and ears with one wipe.” (Related by Ahmad and Abu Dawud.) In one narration it states, “He wiped the inner portion of his ears with his index finger, and the outer portion with his thumb.”