Albert Barnes Commentary Luke 1:18

"And Zacharias said unto the angel, Whereby shall I know this? for I am an old man, and my wife well stricken in years." — Luke 1:18 (ASV)

Whereby shall I know this? The thing was improbable, and he desired evidence that it would take place. The testimony of an angel, and in such a place, should have been proof enough; but men are slow to believe the testimony of heavenly messengers. As a consequence of not believing, he was struck dumb.