"Alive" (生きている, Ikite iru) is chapter 602 of the original Naruto manga.
Disoriented, Obito asks Madara whether he was also an Uchiha after seeing his Sharingan, then after seeing his scythe, whether he was a Shinigami who had come to take him to heaven or hell. Obito panics pleading to the thought-to-be Shinigami to spare him from hell because throughout his life, his actions had not been all that bad. As the young Uchiha winced in pain from his sudden movement, Madara tells him to take that pain as a sign that he was still alive, however barely. Madara then tells the young man that it was he who saved him after he appeared in his underground passage, but in return, Obito must repay him his debt. Obito, however, tells the old man that he can't stay too long for he still had friends to protect as a war was still ongoing. Madara chastises Obito, telling him that with his body in the current state that it was, he could no longer be a shinobi. Dismaying, that he had finally awoken the power to protect his comrades, Madara lectures Obito about the reality of the shinobi world and all its ills. Obito then notices the change in demeanour the old man had, though Obito is still adamant that he wants to leave leading Madara to state that he was free to leave, if he was able to do so. Obito then begins to wonder what an old man with a Sharingan was doing in a place such as this. He finally questions the elder on who he was, asking whether or not he was a missing-nin. Madara, now seated, however, responds stating that he was "a ghost of the Uchiha": Madara Uchiha. In shocked disbelief, Obito states that the man before him could not be his ancestor as Madara had died during his fight with Hashirama Senju decades ago. The elder explains to the young Uchiha that he escaped death and that he needs constant chakra supply from the Demonic Statue of the Outer Path to survive or he would die right away. Aggravated, Obito forces himself out of bed stating that he would go back to Konoha. Madara then warns him that moving will probably rip off the artificial body which he had to use on him. Madara repeats again that Obito needs to do what he says from now on. Obito angrily asks what Madara wants to which he simply replies that he wanted to create a perfect world, but Obito is still adamant, determined to protect his friends. Still weak, Madara explains to Obito that he will take his Sharingan if he dies trying to force himself to leave, noting that he himself was using another's eye, having left his own to someone else and the Sharingan requires two eyes to bring out their true potential. Thinking to himself, Obito states that he can't waste time, and tells Kakashi and Rin to wait for him as he was still alive.