One Piece Chapter 1, page 49-50: Romance Dawn

Sometime later, Shanks' pirate ship is fully loaded and preparing to set sail from the harbor. Shanks stands on the dock, talking to Luffy: "We've been using this harbor as a base of operations for a long time. Maybe too long. Are you sad?" Luffy smiles bravely: "Yeah, I'm sad... But I won't ask you to take me with you! I've decided to become a pirate on my own!" Shanks sticks his tongue out: "There's no way you can become a pirate! It wouldn't do you any good. You're still too little!" Luffy yells back: "Oh yes I will!!! One day I'll have a ship and crew better than yours!! And we'll have the biggest hoard of treasure in the world!!

Shanks looks at Luffy, surprised, then smiles gently: "Hmm... You're gonna be better than us, huh?" He takes off his trademark straw hat and places it firmly on Luffy's head, shadowing the boy's eyes. Shanks says: "Well then... Do me a favor... Keep this hat safe for me?