Lee's Hidden Strength: Forbidden Secret Jutsu!

is episode 49 of the original Naruto anime.

Synopsis
Lee's target was actually only a decoy - a hollow clone made of sand. Gaara reappears and fights Lee, who is now weak from using his Front Lotus again. Lee quickly recovers this time, demonstrating his ability to open the Second Gate. He then powers himself up by opening the Third and Fourth Gates, frightening even Gaara.