Rotating object on z axis only when following mouse movements.

This is the script i am using currently;

 void Update()
 {
     Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
     transform.rotation = Quaternion.Slerp(transform.rotation, Quaternion.LookRotation(ray.direction), speed * Time.deltaTime);
 }

When using this it also effects the X axis rotation, when all i want it to change is the Z axis. Think of it like a tank turret only rotating around, that's the effect i want.

Any help is appreciated. :)