DB Chapter 387
From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
"The Balance of Power" (超パワーのバランス, Sūpā Pawā no Baransu; Literally meaning "The Balance of Super Power") is chapter 387 of the Dragon Ball manga.
Trunks finishes powering up, and says this is his real power. Cell says, "Indeed. Certainly that's splendid 'power'. Fully greater than my own. But, with that, you cannot win against me. Definitely not." Trunks thinks he's just bluffing, and charges at Cell. Up at God's Palace, Piccolo comments on how his ki is incredibly, far greater than Cell's. Bulma asks who, saying if he won't watch quietly by himself, he should explain. He tells her it's Trunks, and she gets excited. In the Room of Spirit and Time, Gohan is a Super Saiyan, with much shorter hair now, and he's out practicing. Goku, sitting down, looks up at him, a bit surprised. Gohan takes to the air, thinking his father has just been sitting there like that every day, and he figures it must be really hard to go beyond Super Saiyan.
Gohan lands, de-transforms, and catches his breath. Then Goku stands up, and turns Super Saiyan. He powers up, until he bulks up to the level Vegeta was at. Gohan is amazed that he's already surpassed Super Saiyan. Goku says it's possible to transform to one more stage. He powers up again, getting even more bulky, to Trunks' level. Gohan says he could definitely win against Cell like that. Speaking of Cell, he dodges a punch and a kick from Trunks, and then another punch. He keeps dodging all of Trunks' attacks, and then lands. Goku says you can't win with this. To swell up this much, your muscles and power get really big, but it kills your speed. It also uses too much energy. For balance, regular Super Saiyan is best. Goku says they'll try always being Super Saiyan, except when sleeping, and that'll be the basics of their training. Trunks wonders why he can't hit him. Cell says Trunks is really green, and that transformation just relies on power. Then Cell bulks himself up too, saying how easy it is.