EXCLUSIVE

Wiz Khalifa was arrested early Sunday morning at an airport in Texas ... after a "green leafy substance" was found during a TSA check.

The "Black and Yellow" rapper was flying from El Paso to Dallas when TSA officials discovered the green stuff ... according to law enforcement sources. Wiz was taken into custody and turned over to local police.

We're told cops concluded the obvious -- the leafy substance was marijuana -- and that's against the law in those parts ... so he was arrested.

Cops say he was in possession of 0.5 grams of weed.