"Sparring" (手合せ, Teawase) is the 17th chapter of the One-Punch Man manga series.
Genos is checking his and Saitama's new rankings in the Hero Registry, claiming that they'd been both put in the lowest rank of the S-Class (Genos) and C-Class (Saitama). After a brief discussion, it turns out that the two have gone to a quarry to spar. Genos asks Saitama to take the spar seriously and then takes a stance.
He holds out an arm, which opens up to reveal multiple cannons, which surprises Saitama. Genos fires a massive beam towards Saitama and then launches himself after the beam. Saitama easily sidesteps the beam but gets caught by Genos which causes the two of them to go flying into the wall of the quarry, shortly followed by the beam. Genos then moves along the side of the wall of the quarry, throwing a flurry of punches until he notices that Saitama isn't there and he'd just been attacking Saitama's after image the entire time.
After realizing that he hadn't actually been attacking Saitama, Genos scans for him and locates him jogging away across the floor of the quarry and launches himself after him. Genos lands in front of Saitama and puts his hands together, both of which open up, and launch a massive blast. Genos thinks he'd managed to get Saitama, but Saitama pats Genos' shoulder and then pokes him on the cheek. Genos then throws a punch at Saitama, who dodges, and accuses him of not taking the spar seriously. At that point, Saitama gets rather close to Genos, who attempts to kick at him, but Saitama dodges behind Genos and throws a punch, stopping just before Genos' face, instead flicking him on the forehead. As Saitama walks away, Genos looks behind him to see a massive dust cloud caused by Saitama's punch.