From episode 5 of One punchman.
After Saitama and Genos have both passed the National Superhero Registry exam, they were placed last in their rank, with Genos being Rank 17 in S-Class and Saitama Rank 388 in C-Class. Later, Genos thanked Saitama for accepting the request for a sparring match and asked Saitama to take it seriously. The motivation for this is to figure out Saitama's
Genos launches himself at Saitama but misses, he then follows up with a barrage of punches which Saitama dodges. He then attempts to hit Saitama with his incineration canon but misses. Genos tries to hit Saitama with another barrage of punches, but is speed blitzed by Saitama as it turns out he was only chasing his afterimage the entire time. Genos tries to hit Saitama with a more powerful than normal version of his incineration cannon, but Saitama easily evades it; taps Genos on the shoulder, pokes him in the cheek when he turns his head, and says that he wins. Genos retaliates by trying to punch Saitama, which Saitama avoids by jumping over and behind him.
Genos then reminds Saitama that the rules of the fight involve him taking the fight seriously and continuing to fight until he is no longer able to fight. Saitama's legs tense slightly and he is suddenly in front of Genos. Genos tries to kick Saitama; only for Saitama to flash step behind him. As Genos turns to face Saitama's fist hurling towards him he feels as though he is going to die. Saitama stops his fist just before it hits Genos and all the force of the blow seems to just travel around Genos. Saitama taps Genos on the forehead and says they should eat some udon.
Saitama's punch is revealed to have blown a hole through several mountains and clouds and Genos thinks he will never become even close to as strong as Saitama.
