Ep. 28: Delay Majin Buu, The Limit! Super Saiyan 3!!

28

Realizing that they won't be able to find the Dragon Balls if the Dragon Radar is destroyed, Goku orders Trunks to head to West City to retrieve the Dragon Radar from Capsule Corporation, while he heads off to delay Buu and Babidi. In order to buy Trunks some time, Goku builds up a large amount of energy, which also uses some of his remaining time on Earth, to transform into his ultimate form, Super Saiyan 3, with its immense power being felt even in the world of the Kais. (Source: Wikipedia)