This episode is episode 155, the first challenge. This is where fukasaku is teaching Naruto the first steps to be a sage. The training is called "don't move" and he has to focus his natural energy to becom a sage.
Naruto learns sage mode by chanelling the flow of senjutsu chakra or natural chakra into the body. He learns to use it on Mount Miyaboku after Jiraiya dies.
This episode is episode 155, the first challenge. This is where fukasaku is teaching Naruto the first steps to be a sage. this training for his fight with pain
Naruto learns Sage Mode after Lord Fukasaku, informs Konoha of Jiraiya's death at the hands of Pein. He then tells Naruto to come with him to Mount Myoboku to be trained by him in the sage arts, because it was originally Jiraiya's plan. Naruto Shippuden Episode 154.
when gamutatsu puts the magical dildo in in his bum and he instantly becomes sage mode and the 9 tails is released whilst sasuke is getting killed by sakura