believe one's ears - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Also, believe one's eyes. Trust one's own hearing or sight, as in We couldn't believe our ears when we heard that Gene was accepted at Stanford, or I couldn't believe my own eyes when the movie stars walked in. This expression of incredulity is nearly always stated negatively, as in the examples. [Early 1600s]