"The First Challenge" (第一の課題, Daiichi no Kadai) is episode 155 of the Naruto: Shippūden anime.
Naruto, now at Mount Myōboku is informed by Fukasaku that he'll face the risk of becoming a stone frog if he takes in too much natural energy but he then assures him there is no need to fear since his staff has the ability to knock the natural energy out of anything as long as there isn't too much intake. To expedite the process of becoming a sage, Fukasaku rubs some oil from a fountain on Naruto so he can take in natural chakra and become familiarised with it before the real rigorous training begins. Naruto suggests the idea of using shadow clones to expedite the training process. After hard work and revelation that only those with high chakra reserves like Jiraiya and himself could undergo senjutsu training, Naruto finally begins to make progress making a slight but acceptable transition into frog form. He lifts a frog statue and sadly, topples it which Fukasaku scolds him for. Naruto begins having doubts that he isn't becoming less human by the hour and goes to eat. Fukasaku later finds Naruto and gives him Jiraiya's first novel, The Tale of the Utterly Gutsy Shinobi. Naruto reads it and after becoming emotional, he reminisces on a time where he and Jiraiya had a talk about world peace. The scene skips into the extraction of the tailed beast where Tobi, Zetsu and Kisame realise that Sasuke has blundered and only a transformed appendage of the Eight-Tails. Pain later sets out to invade Konoha and procure the Nine-Tailed Demon Fox.