Book/Source: The Hobbit
Disclaimer: I don’t own Middle-earth, however much I’d like to live there.
"...and then Gandalf appeared with a bright flash in between the armies - " A burst of lightning accompanied by a loud boom of thunder interrupted Bilbo's story, making several of the children jump and shriek, as though Gandalf at his most wizardly had appeared there in the room with them.
Bilbo laughed. "Yes, it was much like that! And now pay attention, for we are approaching the most exciting part..." It was lucky such a tempest had descended on them that evening - stormy nights were perfect for storytelling, especially stories about goblins and wizards and great battles over dragons' gold!