I have no idea how to prove this. By assuming the field has the curl I get these 3 equations:
$$x = \frac{\partial F_{3}}{\partial y} - \frac{\partial F_{2}}{\partial z}$$
$$y = \frac{\partial F_{1}}{\partial z} - \frac{\partial F_{3}}{\partial x}$$
$$z = \frac{\partial F_{2}}{\partial z} - \frac{\partial F_{1}}{\partial y}$$
I cant't see how to get a contradiction from here.
Hint: If one vector field is the curl of another, what is its divergence?