In which episode did Luffy say the following quote?
It's from Episode 236: this is a clip from the episode at the scene you mentioned.

The context is the duel between Usopp and Luffy: after Luffy decided to buy a new ship and abandon the Going Merry, Usopp abandons the crew and challenged Luffy to a duel. The cross talk is between the two of them just before the duel began.