Ohnoki's Thoughts, Ku's Thoughts Boruto: Naruto Next Generations : Season 1 Episode 84

Boruto and Ohnoki head to the rendezvous point, but they stumble into the Hidden Stone Shinobi Training Grounds, which are protected by a barrier. Ohnoki tells Boruto that the only way out of the Training Grounds is to find his heart of stone, something that can only be found once in a lifetime when a person comes to terms with their will.