You Definitely Will (お前は必ず, Omae wa Kanarazu) is chapter 687 of the Naruto manga.
Kakashi asks the crumbling Obito why he would sacrifice himself when he was still necessary in battle, but the silent Uchiha muses that Kakashi was the one needed to support the next generation, before telling his comrade to focus on the enemy. Kaguya decides to return to her first dimension while she replenishes her chakra, but when Naruto tries to use his abilities to save Obito, Black Zetsu launches a tirade not only about Naruto's futile efforts, but what he believes to be Obito's wasted life; and though Obito agrees to this, Zetsu's words do nothing but infuriate the touring Uzumaki. Wasting no time when he realised that his dōjutsu's ability was useable once more, Sasuke launches an attack on Kaguya, forcing her to shift dimensions once more to narrowly escape the attack. Noting that this time he would be the distraction, Sasuke engages Kaguya giving her no time to relent as he dons his Complete Body — Susanoo and flies towards her. Obito uses this time to impart his final words to Kakashi and Naruto, thanking them for allowing him to remember who he used to be. After telling Naruto to become Hokage, Obito finally disintegrates into a pile of ash, leading Black Zetsu to comment that Obito was a cockroach for having last so long rather than a regular insect. Enraged at this point, Naruto declares that because he wanted to become Hokage at one point, Obito was an awesome person in his eyes, and proceeded to fly up to Kaguya at an extreme speed, severing her left arm with his own chakra arm. Meanwhile, in what appears to be the afterlife, Obito finds Rin and the two reconcile.