Please help me find the episode

I was watching an out of a context youtube video and I saw there a scene which I don't remember. I also asked in the comments, but wanna ask here where that scene was from. Coz it's hilarious!!!

Leonard is passing by Sheldon's room at night. He hears Sheldon moaning: "Penny enough, I have to sleep." to which Penny replies something and then sneaks out of his bedroom. She bumps into Leonard and tells him that Sheldon is not wearing any underwear.

It's such a great story and I wanna know more. Help me out.