DB Chapter 312
From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
"50% Maximum Power" (怪物フリーザ, Kaibutsu Furīza; Literally meaning "The Monster Freeza") is chapter 312 of the Dragon Ball manga.
Freeza says Goku is strong, but he's getting tired of this fight. Before he ends it though, he wants to ask Goku if he'll work under him. He thinks it would be a waste to destroy such a strength that's greater than even Captain Ginyu. Goku asks Freeza if he really thinks he'd take such an offer, and Freeza had thought not, since Saiyans are all stupid. And he, as the last remaining Saiyan, must die. Goku says it won't be that easy. Freeza thinks he's pretty self-confident, and says he's realized that while Goku's fighting seriously now, he still has quite a bit of power left over. Though Freeza figures that if he used 50%, or half, of his maximum power, he could turn Goku into space dust. Goku thinks he must be exaggerating. Freeza says this has been fun, and it's been a really long time since he's had this much exercise.
Piccolo says they're both looking for the time to attack, and goes on about how both Goku and Freeza have long since surpassed their (Piccolo and everyone's) comprehension. Freeza and Goku stare each other down, and Goku suddenly realizes Freeza isn't bluffing. Freeza charges at Goku and elbows him in the face. Just as Goku regains his footing, Freeza sweep-kicks him, grabs Goku's neck with his tail, elbows Goku in the stomach, and then drops him. Goku clutches his stomach, and tries to kick at Freeza, but Freeza simply jumps away. Goku charges after him, but Freeza kicks Goku in the face, sending him flying. When Goku finally gets back on his feet, Freeza notes that Goku's finally starting to breathe hard, though it's amazing he hasn't died yet.
Piccolo thinks the difference in their hidden strengths is too great. Meanwhile, Kaiō thinks it's all over, since Freeza's just far too strong. Yamcha asks if Goku is still wearing his heavy dōgi, but Kaiō says he's just been wearing regular clothes the whole time. Freeza punches Goku in the face, as Tenshinhan says there's nothing to worry about, and that Goku will win. He figures Goku must've forgotten about Kaiō-Ken, and Goku as he is now could probably use a 10-fold Kaiō-Ken, and Yamcha agrees. But Kaiō says, "Unfortunately... He's been using the 10-fold Kaiō-Ken..."