Watch your leg

John : (sits inside a taxi and says to Peter) See you later.

Peter : (holds the taxi door and is about to close the taxi door and says) See ya. Watch your leg.

* Hello native speakers. Peter doesn't want the taxi door hitting John's leg so he says "watch your leg". Between "watch your leg" and "watch out for your leg", which phrase sounds more natural in this context?

Thanks very much in advance for your answers.