What did Ponyboy say that caused two bit to slap him across the head while cherry and Marcia looked on?

Ponyboy didn't say anything to cause Two Bit to slap him. It's the other way around: Two Bit slaps Ponyboy in the back of the head after Ponyboy asks him a question about why he's always trying to get in trouble.

The specific scene occurs in Chapter 4 of *The Outsiders*, when Ponyboy, Cherry, and Marcia are at the drive-in. Ponyboy is curious about Two Bit's rebellious nature and asks, "Why do you always get in trouble?" Two Bit responds with a playful shove and the slap, saying, "I don't always get in trouble, see? You just happen to be around when I do."

The interaction is more of a playful banter between the two friends than a serious conflict.