It's a Promise!! (約束だ!!, Yakusoku da!!) is chapter 180 of the Naruto manga.
Lee is indebted to Guy for giving him hopes of achieving his dreams of becoming a ninja and guidance in fulfilling those dreams. However, Guy's advice of hard work will do him no good with the operation that will restore him to fighting capability, as his survival is a matter of luck rather than skill or persistence. Because his death would leave him unable to be a ninja in any capacity, he is torn over what to do. Guy tells him to go through with the operation since Lee would only be even sadder if he gave up on his dream. If he at least tries to resume his life as a ninja, that will make all the difference and he will definitely survive. Lee is still reluctant, so Guy tells him that if Lee died in pursuit of his dreams, Guy would die too for being unable to fulfil his own dreams of helping Lee achieve his dreams. Lee decides to go through with the operation.