Other source:
Buu gives Goku some time to find someone to fuse with. He considers Dende and Hercule, but neither of them would be any help. He suddenly senses that Vegeta has returned, and uses Instant Transmission to find him. Dende then heals Tien. After he finds him, Goku tries to convince Vegeta to fuse with him, but Vegeta is angry because he knows Goku was holding back his Super Saiyan 3 transformation when they fought. They each try to take on Buu, but are clearly outclassed. Goku tells Vegeta that Buu absorbed Trunks and killed Bulma and asks him to put their differences aside to save their race. Vegeta finally agrees, and only when he puts the earring on does Goku mention that the fusion is permanent.
