In Episode 916 of One Piece , titled "A Life of Purgatory! Luffy’s Humiliation in the Great Prison!", Luffy is sent to a work camp in Udon, while his allies escape Kaidou’s attack and learn of his capture. The episode also reveals that Eustass Kid is imprisoned in the same facility, and both pirates refuse to break, setting the stage for more conflict.
You can watch the English dub of this episode on Crunchyroll or through BBC iPlayer. One Piece (Dub) Episode 916