believe one's ears
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]
Dictionary.com presents 366 FAQs, incorporating some of the frequently asked questions from the past with newer queries.