DB Chapter 174
From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
"Yamcha vs. Shen" (ヤムチャ対シェン, Yamucha Tai Shen; Literally meaning "Yamcha vs Shen") is chapter 174 of the Dragon Ball manga.
Now, it's time for the fourth match, and Shen nervously heads out to the ring. Kuririn tells Yamcha he's lucky, and then Yamcha heads out to the ring, thinking this'll be quick. Shen tells Yamcha to be careful with him, and so Yamcha (jokingly) tells Shen it should be the other way around. Yajirobe, out in the audience, thinks how this Yamcha dude is Son's friend, and he's going to lose pitifully. Goku thinks there's something up with this Shen guy as well.
"Well then, please begin the fourth match!!" Yamcha (who isn't taking this seriously at all) tells Shen to come on, and Shen (who's in a nervous/jolly mood) says he's going to attack him. Shen starts flailing his arms, and then runs at Yamcha like a goof, and Yamcha ducks his swipe. Shen trips and falls over, accidentally kicking Yamcha in the gut. Yamcha laughs nervously about that accident, and Shen nervously says that was a shame. Yamcha gets pissed and thinks it was really a shame for himself, and he wants to end this quickly.
Yamcha, who's stopped being serious again, says he's going to attack now, telling him to please get ready. Shen takes a goofy stance, and Yamcha thinks he'll just knock him out of the ring. Yamcha jumps at him with a kick, but Shen quickly ducks, and Yamcha ends up hitting his crotch on the back of Shen's head. Yamcha falls to the ground clutching his balls as the audience cracks up with laughter, with Bulma being really embarrassed.
Yamcha is really pissed now, as Shen nervously apologizes. Shen then says he'll win either way, but this way was no fun. Yamcha asks if he really thinks he can beat him, and Shen says he does. Yamcha says he'll get serious as well, and Shen says he'll go now. Shen then hits Yamcha with an elbow, then starts lecturing Yamcha about his wasted movements once he gets up. Yajirobe knew he wasn't ordinary. Yamcha is shocked, and then gets serious and comes at Shen. But Shen blocks all of his punches, and then kicks Yamcha off his feet.