One Piece Chapter 1, page 45-47: Romance Dawn

The sea monster turns its attention to Luffy, who is desperately splashing: "Glug! Somebody help me!! Glug! Glug!" The monster lunges forward, its massive jaws wide open to swallow the boy. Just as its teeth are about to close, Shanks appears in the water, grabbing Luffy and pulling him close to his chest.

Shanks holds Luffy tightly and glares at the giant sea monster with an incredibly fierce, terrifying look. He commands: "Get out of here!" The monster freezes, sweating in pure terror under Shanks' intense gaze. Whimpering, the beast turns and quickly swims away, splashing back into the depths.

Shanks floats in the quiet water, holding Luffy. He says gently: "Luffy, I'm sorry. Makino told me what happened. You stood up for me." Luffy begins to sob uncontrollably: "Shanks!!! B-but..." Shanks smiles warmly: "Hey, stop crying! You're a tough guy, remember?" Luffy looks up, crying hysterically: "WHAT ABOUT YOUR ARM!!?" Shanks' left arm has been completely bitten off, bleeding heavily into the sea.