For a Friend… (親友よ!, Tomo yo!) is episode 132 of the original Naruto anime.
Kakashi learns of Sasuke's defection and immediately goes after him after trying to reassure Sakura. Meanwhile, believing Naruto to be his closest friend, Sasuke prepares to kill him, finding the loss of Naruto to be an acceptable price for the Mangekyō Sharingan. Naruto's Rasengan and Sasuke's Chidori clash, though neither is capable of overpowering the other, and they both are sent back flying. Naruto begins to cry for his friend silently, then Sasuke states that if two ninja are of a high enough level, they can read each others by exchanging blows and asks Naruto if he can read his mind. Activating his cursed seal in order to gain the upperhand, Sasuke uses Fire Release: Phoenix Sage Fire Technique and distracts him. Sasuke beats Naruto, who would appear to be putting next to no effort at all in defending himself. He has brief flashbacks about how he was happy to meet him and how they became rivals and wonders if they ever truly had a bond. Sasuke uses his Chidori to attempt piercing through Naruto's heart, but it is deflected and it goes clean through Naruto's right lung instead. While Sasuke thinks the battle to be won, the Nine-Tails' chakra begins to envelop Naruto and the wound Sasuke inflicted starts to heal. When Sasuke asks what he is, Naruto tells him that he's his friend. Infused with the fox's strength and now willing to break Sasuke's body in order to bring him home, Naruto attacks.