Neither; it should be 'Why are you asking ?' You will hear 'Why you asking ?' in casual conversation but it's not grammatically correct.

"Why you asking?" is the more informal and grammatically incorrect way to ask the question. You will hear it in very informal, casual, everyday conversations. The correct and more fluent way to ask is: "Why are you asking?"

Why are you asking

Or “why do you ask?”

Neither. It would be “Why are you asking?”