Bushy Brow's Pledge: Undying Love and Protection!

is episode 31 of the original Naruto anime.

Synopsis
While hiding under a tree, Sakura tended to Sasuke and Naruto's wounds, even though severely sleep-deprived. Alerted from a daze by a squirrel, Sakura quickly wards it off with a kunai ignorant to the fact that Team Dosu had sent the squirrel there with an explosive tag on its back. While en route to find a second scroll, Rock Lee stopped on a branch, and sets himself the task of catching all the leaves that had fallen, which if he did, Sakura would fall in love with him. After catching 19 of the 20 leaves he saw a squirrel with an explosive tag on its back, and saved it before destroying the explosive tag. Dosu and his squad challenged Sasuke, but Sakura, prepared to defend her comrades set off several traps at the enemy shinobi. Unfortunately Dosu managed to destroy the trap with ease, and shortly after that made a remark about how Sakura had no right to be a ninja. Just before they could land their attack, however, Rock Lee showed up and knocked the three of them away. Declaring as he made his entrance that he would protect Sakura until he died, Lee uses brute strength to pull up an underground root to block and incoming attack by Dosu. As the battle ensued Lee prepared to use the Front Lotus after deeming that it met the criteria of protecting a precious person that his teacher, Might Guy told him had to be met in order to use it. Lee is then able to catch Dosu and use the technique but the force of impact is cushioned by Zaku.

Trivia

 * The task Lee sets for himself is very similar to the game often played to discern whether or not the object of ones affections returned the same feelings.