Two Heartbeats: Kabuto's Trap

is episode 140 of the original Naruto anime.

Synopsis
Naruto arrived just in time to save Sakura, but not before Kabuto told her that Sasuke was dead. Naruto told her not to believe him, after which a battle ensued. Naruto managed to defeat him with the Rasengan, but Kabuto trapped him with some chakra threads that connected their hearts; if Kabuto's heart stopped beating, so would Naruto's. As Kabuto attempted to kill himself in order to end Naruto's life, Sakura tried to free him, but only the special threads are unbreakable. Sasame arrived and gave Sakura a special Fūma clan tag to help save Naruto, only to find out that the Kabuto they had fought was actually Kagerō, who died after suggesting that Sasame should not know what Arashi had become. They finally found Arashi, who had been disguised as Orochimaru, who revealed that he had indeed changed.

Trivia

 * Back in February 16, 2008 on Cartoon Network's Toonami, this was the only Naruto episode to receive a parental warning.