Place it immediately after the unbelievable claim in an edit, then follow it with Bruh for a second beat, or put Oh My God Meme before it to build the reaction in stages before the question arrives.
Über diesen Sound
This is a short spoken clip of a voice asking 'I'm sorry, what?!' with sharp, incredulous emphasis, lasting about six seconds. It lands as sudden disbelief mixed with confusion, the sound of a brain stopping mid-sentence, and it works because the question feels both polite and completely thrown.