It's not a travel unless he moves his pivot cheek.
Actually, the points made in the thread are really interesting. By strict interpretation of that rule, it would be a travel if the player touched the floor with his back or his elbow, or even his wrist, if you don't consider the wrist a part of the hand. I must admit in all my years, I've never called a travel if the player sitting up while on the floor with the ball laid back down, and certainly not if his elbow hit the floor. Maybe I was wrong.