One Piece Chapter 32, page 11-13: Grave Danger

Watching the struggle, Nami worries, "OH NO! ZORO CAN'T HANDLE THEM!" Usopp decides to intervene, saying, "HE CAN'T COUNTER WITH TWO PEOPLE ATTACKING HIM!! I'M GOING TO HELP HIM!" Nami tries to stop him, asking, "WHAT ARE YOU DOING?!" but Usopp fires his slingshot anyway, shouting, "EAT THIS! 'NAMARII-BOSHI'!!" (Lead Star) toward the combatants.

The lead star flies directly toward the fight. Zoro notices the projectile and is forced to react, distracting him from his defense. Sham and Buchi instantly seize the opening, shouting, "HE'S DISTRACTED!!" Zoro grunts in dismay, while Usopp realizes his mistake, gasping, "AH?!

With Zoro's guard broken, Sham and Buchi slash him across the back. Zoro screams, "UWAHH!!!" and falls heavily to the ground, bleeding. Usopp is horrified, crying, "NO... NO...!! HOW COULD YOU HIT HIM!?" Nami, however, realizes the truth, explaining, "HE DELIBERATELY BLOCKED IT WITH HIS BODY...!!" Usopp is stunned, stammering, "HE DID THAT...?!